it would appear my initial concerns are confirmed with a bonus prize. Earlier I stated it seemed odd that you had stat blocks up to CR 4, then boss monsters with CR 21+, and nothing in the middle. The notion that WotC compensated for this by referencing specific encounters like MM creatures was also delusional. CR 4 encounters are about right for a "not quite level 8" group. The sunsword is incredibly powerful for a level 1-4 adventure. Vs. Orcus it's like a Varmint Rifle vs. an Elephant, and there's more than 1 Elephant in the room. I think one of the Demon Lords, Orcus, you might be allowed to consider him +1d8 radiant damage as an Undeath Demon, but in RAW, maybe not. That leaves you with a 1d8+2 weapon. Adding your token +5 from Dex or Str, and you have 1d8+7 (12).
Assuming 75% of the attacks hit, assuming 3 attacks per round, You are looking at 27/round. The demon lords are around 400 hp each with variation. So if you can live 10 rounds, then sure. Radiant, Force, and Acid spells are probably your best bet, with automatic saves and half of half being quarter damage, so about 10 radiant damage per additional spell caster each round. A group of four yielding possibly 57/round gets you about 7 rounds on a good day. Not sure how many PCs the demon lords can take down in 7 rounds, but that's what goons are for. But notice something odd?
Your resources aren't intended to function any better than hirelings, random NPCs and so forth. Basically, if the game is designed as a suicide mission where the plot defeats the enemy, then the DM can run this game with just themselves and a party made up of their favorite character and some hirelings. That was my original conjecture. The facts that 1. the storyline assumes goons are provided and 2. the DM has to basically pencil in 80% of the experience points from random encounters means it's true. This is a product designed for the DM's own pleasure.
DMs buy adventures, and Players buy supplements for their characters. DMs run adventures and the players enjoy being around the DMs, but I think this adventure is relying heavily on two appeals. 1. Appealing to the DM, making it fun and work for them, to the point that they don't even need actual living players to run it, and if the living people do show up, its so much railroading or stacked against the PCs that the DM can get a huge ego boost; 2. Depends heavily on the DM's personal charisma to drag people through a very unpleasant sequence of sadistic events. The Table Top Equivalent of Cut Scenes and Broken Minigames.
I have a couple of Charismatic Dungeon Masters so they will probably try to make this work. One of them will probably just use the auto-leveling milestone system like they did with Tiamat. But this Adventure has the mark of the Realms on it for sure. You can always tell when the importance of the PCs is so heavily de-emphasized and the NPCs and Villains are the most important part of the story.
Here's some questions.
1. you know you need a whole bunch of radiance spells or preferably enchanted weapons to face one of the bosses (since the others are going to WWE themselves down to 1) do the wizard characters have even the 10-20 days required downtime to manufacture a +1 weapon?
2. Has anyone else realized it might be really, really dumb to go from level 1 to 15 in less time than it takes to enchant +1 weapons? None of the hirelings have them, from what I recall, so they are all just meat bags.
3. What if you don't want to watch an epic cut scene of WWE demon lords tag team matching Jubliex and putting Demogorgon in a figure 4? What if you want to take your adventuring party against all of them, either one at a time, or as a group, is that even possible in 5e? In other words, are 5e characters forever doomed to be mere shadows of AD&D PCs?
In my imagination, I see epic heroes clashing against titanic forces of darkness. But I don't think the adventure is designed that way. I think we are supposed to bring pop corn, be awed, and then towards the end of the cage match, fling the bodies of our golf caddies at them like T-shirt cannons, and then hose them down from range after they've been softened up from too many DDTs by Graatz.