Oh! I can play this game. Gandalf needs only have a nuclear bomb and set if off, they both die and he can be 0 level. There's nothing that says the staff is anything more than a focus for Gandalf's spells.
I agree that there is nothing that says that Gandalf's staff is anything more than a focus for Gandalf's spells, and said so in my write up. But it is important to note that the original essayist did not make that assumption. Rather, the original essayist and many of his readers would have assumed that Gandalf has a Staff of Power. The essayist mentions that on account of the staff and the ring, it would be possible that Gandalf is less than 5th level, since both would have in and of themselves explained Gandalf's ability to cast the spells he does. Certainly it would appear to me that the idea of the retributive strike comes from the scene of Gandalf breaking his staff on the bridge of Khazad-dum, although I would have to check 'Playing at the World' to verify if this is true. Nonetheless, this gives plenty of ability for a player from a oD&D or AD&D perspective to imagine a 5th level Wizard beating a Balrog, and that is sufficient along with all the other evidence to suggest Gandalf could have been a 5th level Wizard.
It's self-evident why it is more reasonable to assume Gandalf has a Staff of Power rather than a nuclear bomb.
He can't be 6th level. He was instructed in Aman not to use his full power, so if you are correct and he used his full power, he's a fallen Maia who rebelled against Aman. Since we know that isn't the case, he's has to be more powerful than the magic that he shows.
Once again, you are misremembering the text and relying on a loose summation of a passage rather than the text itself. They were commanded not to use their power to dominate the free peoples, nor to attempt to overcome Sauron's power by power. Gandalf normally conceals his full power as a 6th level wizard so as to not overawe the free peoples, because in Tolkien's world a 6th level wizard is incredibly powerful awe inspiring worker of miracles capable of doing things far beyond what is possible for ordinary people, but Gandalf is perfectly free in a pinch or in private or when away from observation to use his full power in self-defense or to protect those same free peoples provided his purpose in doing so is not dominating the free people, nor with the ultimate aim of defeating Sauron by his own power. When Gandalf does his fireball type thing against the phantom wolves, or uses lightning bolts against the goblins, or both against the Nazgul on Weathertop, there is no reason to suppose that he is not using his full power. After all, Gandalf rarely chooses to do even that much. And there is certainly no reason to suppose that Gandalf is breaking the command of the Valar to do so.
Gandalf differs from Saruman not in that Saruman had decided to reveal his full power as a 18th level wizard (or some other crap), because there is certainly no evidence Saruman had any such power, but in that Saruman had decided to rule over the free peoples and defeat Sauron in a direct contest of power. Saruman himself was probably not more than a 7th level wizard, plus whatever other racial powers Gandalf had, themselves no more potent than what would be expected of a character of about 6th or 7th level. Any big difference in the power level of Gandalf and Saruman appears to be related to what magic items they each had access to. Gandalf of course had Narya, as a gift from clear eyed Cirdan, and this is probably the ultimate source of Saruman's jealousy and eventual fall. Saruman finds the Palantir in Orthanc and so has powers of observation and action at a distance that Gandalf lacks. But neither acts in the way a high level wizard in D&D acts, despite the fact that Saruman is disregarding the commands of the Valar. That in itself ought to be sufficient to show that when Gandalf is concealing his power, it's concealing that he's a 6th level wizard and an ainur, and not that he's concealing that he's an 18th level wizard.
Now, there is an important point to be made here. Fireball is actually more powerful than anything Gandalf can actually do. The text makes perfectly clear that Gandalf cannot make fire in midair or set alight anything that cannot burn. There is good reason to suppose that fireball is a more powerful spell than anything Gandalf can actually do on his own. However, as with almost everything else in D&D, it's both absolutely certain from the historical evidence that the spell Fireball is intended to represent the power of Gandalf, and also its equally clear to any Tolkien scholar that just like almost everything else ported from Middle Earth to D&D it's a pretty poor translation. So while a very faithful Middle Earth RPG probably wouldn't have fireball in it, within the context of D&D Gandalf casts fireballs. A similar comment applies to Gandalf's staff. While it's pretty darn certain that Gandalf's staff in Middle Earth was not what D&D players call a Staff of Power or a Staff of the Magi, it's likewise pretty clear to me that the idea of the Staff of Power comes from Saruman's banter with Gandalf when he asked if Gandalf wanted to have the Staffs of the Five Wizards and boots of a larger size, and that the retributive strike is likewise an interpretation of why Gandalf broke his staff and what it meant when he did so.
Most of what is in D&D is in one way or the other, all disclaimers aside, inspired by Tolkien's works. Often it is based on the reader's lack of clarity regarding the text or wrong imagination of the scene, as for example with the fireball spell or any number of other things, be it elves (who don't need to sleep), elvish chainmail, goblins, ents, or mithril. To say then that the mithril of Middle Earth is far more powerful than the mithril of D&D is on some level ridiculous. The mithril of D&D is intended to be the mithril of middle earth. And unlike say fireball, there is no reason in the text to believe that it's not a reasonably faithful interpretation of what the text reads. You might be able to reasonably claim that over time mithril has become more ubiquitous in D&D than it was in Tolkien's world, where it was rare in the extreme and so more valuable. But that's a later innovation in how the game is played, not something that existed or was intended in 1977.
To the extent that we know the original essayist got some details wrong, it's not his fault but simply the result of us having the Silmarillion to examine - and he didn't. Thus he could not be nearly as clear on the fact that Gandalf was an angel as we are, and so had no firm reason to think Gandalf was anything other than he appeared to be to the free peoples of middle earth - a wizard, perhaps human, or perhaps once you realized he didn't age, then of some elvish origin (hence "Gandalf", "Staff Elf"). And if you are to assume that Gandalf's powers are only that of a wizard, there is no reason to assume that he is anything more than a 5th or 6th level wizard. And, even if you do know that Gandalf is actually an angel concealing his true form and power, there is still no reason to assume that Gandalf is capable of casting more than 3rd level arcane spells using his own lore. Indeed, the original essayist is quite right to believe that it is possible that Gandalf, at least as it involves fire spells, might not have been capable of that much without having a Ring of Power as Gandalf was not in his native authority a master of fire.
Ancalagon wasn't a 10 hit die red dragon. He was unique and more powerful than Smaug, who might be a 10 hit die red dragon. Ancalagon broke mountains when he fell. No 10 hit die red dragon is going to do that.
Why not? Exactly what a dragon breaks when it falls is pretty much entirely up to the DMs license, and indeed even the term 'broken mountains' involves a bit of literary license to decide what is meant by that. The basic problem you seem to have is that you know that in 3e there are 34HD dragons, and so 10HD doesn't seem very potent to you. But a 10 HD ancient red dragon in 1e has 80 hit points, and saves as and probably should be treated as for the purpose of XP a 17HD monster. And a monster with 80 hit points and effectively 17HD is enormous and epicly powerful in 1e D&D. Keep in mind that the attack table only goes up to 16HD. But ok, even if we assume Ancalagon was a unique dragon and not simply the largest possible size of firebreathing dragon which I don't agree that we should, then he is no more than the Tolkien universe's equivalent to Tiamat - and 1e Tiamat only had like 133 hit points.
Eru only played conductor. The Ainur brought the universe into being and shaped it via their song to the plan of Eru. The Valar doing the brunt of that work. No Solar could do that.
The Ainur did not bring the universe into being with their song. The Ainur brought the idea of the universe into being, although obviously, even this idea was beyond their ability to create on their own. It required Illuvatar's word and the power of the flame imperishable that belongs to Illuvatar alone to make the idea of the universe actually become real. Read the scene again. The world only stops being an idea not when the song is sung, but when Illuvatar declares that the song should become real. As for the ability of a Solar to do the work, Solar's can cast both Wish and Miracle. So yeah, I think it's reasonable to believe that a being that mighty can as a work of labor shape the raw material of the world into form over the course of countless eons.
And in any event, we know from external sources that the Tolkien cosmology explicitly parallels the Catholic faith. As such, there is no reason to suppose at all that anyone but Illuvatar has actual divine ranks, or that the Valar are anything other than explicit parallels to archangels.
They were alive and breathed, even if puppets. They just didn't have minds and will. No Solar could do even what you describe, let alone what actually happened.
A solar can easily cast Animate Object, which is afterall only a 6th level clerical spell, and which would allow an object to appear to be alive - though actually mindless. The SRD says of the spell: "You imbue inanimate objects with mobility and a semblance of life." I believe that it is compatible with the idea of a semblance of life for a statue to move and breathe and appear alive. The Silmarillion text explicitly says that everything the Dwarfs were doing prior to Illuvatar's intervention was simply Aule animating them according to his will, and they wouldn't have moved at all if he had not continually willed it. So if anything, Aule had less power than Animate Object appears to confer, although obviously we can't be sure how Animate Object actually works.
Fundamentally, you are repeating the error the original essayist is speaking out against. The idea that the Valar have to be greater gods or that Ancalagon can't simply be the largest possible firebreathing dragon or that Gandalf can't be a 6th level wizard or that the Balrog of Moria cant' be an ordinary Balor involves mentally diminishing what those things actually are, and making them in your imagination small and trite things. But not only is that contrary to the intention of the original author of those things which intended those things to not only represent the very things you claim they can't be and who likewise intended them to be awesome, but in doing so you are unnecessarily crapping on your own game world by making your PC's small things of little worth, your stories smaller things than the game intended them to be, your setting more trite than it was intended, and your gameplay slower and more complicated than was intended simply because you are insisting on multiplying all the numbers by some factor just to make them feel extraordinarily large to you. But if you look at the text, that's ridiculous. Balors and Solars are intended to be so rare and powerful as to be countable things in the entire multiverse. It's not the original conception that is small - it's what DMs have done with them since then.