Nice theory crafting. Guess we ran things differently. It all depends on party size. Mine were usually around 6 players with a few henchmen. Having two clerics was not that uncommon and would open up the possibility of taking a bless spell or two. A full party of adventurers could be anywhere from 6 to12 people. Most henchmen were falling behind as the group rose to higher and higher levels but as the players were stronger, new strategies would open up and henchmen would go down in usage and would go down to one or two for a high level party.
I just repeat experience. I mean, sure every table did some things differently, nobody is going to dispute that. We had many idiosyncrasies in our play, although we did stick pretty closely to the published rules, aside from a couple of things like training, and maybe being a bit loose with tracking every single minor component expenditure and whatnot (I seem to recall that for a long period of time, probably more in 2e timeframe, we used something pretty close to 4e's "generic power source component" rule).
In terms of henchmen. Getting 1/2 XP is not that big a deal. Given the arithmetic series nature of XP tables you are only 1 level behind 'master' (but maybe 2 since you won't get the best items). That certainly makes such characters reasonably valuable at all levels. Hirelings obviously become useless past 3rd level or so however, though they might serve some clever purposes if you hire 100's of them (IE a small army). I think their main intended use at high levels was more to 'hold down the fort' or carry around the less generally useful spells and equipment that you might want to trot out now and then.
As for party composition, I do not doubt your analysis, but to each his own and sometimes you have to make do with what you got. Not all party are build with optimisation in mind, and the rolls might not go your way.
I would think that most parties were NOT built with optimization in mind. Many were piecemeal assembled in increments. Suitable ability score arrays of course mattered, though we pretty much always used Method I, so you were likely to be a passably good instance of one of the 'big 4' classes at the least. And obviously the various subclasses would appear and factor into things to a degree, though none of them is so radically different that it makes a huge difference. I think the Druid was probably the biggest oddball there, as they are not so good in melee but have an interesting spell load, yet lack healing right off the bat. Adding one to the party at 1st level is probably reducing capability a bit, but they are almost like a MC MU/Cleric past 2nd level.
As for thieves being terrible scouts at low level. Yep. Most thieves were elven. But human thieves and halflings thieves were also a thing and from level 6 an on, they were very good, or at least good enough to make them worthwhile as they rose in level faster than any other classes. Your MU/T quickly fell behind in thieving capacity as you neared level 9. At that point, you mu/t would simply use improved invisibility and haste to backstab any opponent to kingdomcome.
Yes, halflings are another good option, though they lack the +1 to-hit, magic resistances, and good infravision of the elf, a stout can see in the dark pretty well, and they do have a stealth rule, as well as dwarf-like resistances and some 'stonework skill'. Probably the best dungeon thief overall, though elves being able to see secret doors is pretty awesome.
Again, being a multi-class is not that much of a hindrance. You are 1 level behind in each class vs the other single-classed PCs, which is not a HUGE disadvantage. Demi-humans (non-thief ones) are borked at high levels anyway, so the "drag your dead maxed out class along" problem rarely matters much. Certainly in 1e the elven Fighter/Magic User is pretty nearly the ideal character. Obvious reason why 2e tried to nerf them a bit with their armor-casting rule (it worked somewhat).
As for the training rule
We used them. The only mod was that we allowed exp to go up to half the next level. So if you rose to level 2, you could accumulate half the experience for level 3. It was usually enough for everyone.
Yeah, I have seen that rule too. SOMEWHERE there were printed rules of this ilk, I swear. I know I read somewhere a rule about 1XP short of gaining 2 levels at the very least. It might have been some article or was it maybe in UA? Or maybe it was officially a 2e rule that we had just been using for a long time before 2e came out (2e is mostly codification of someone's house rules I have to assume, plus a but of tinkering).
