Your spell knowledge of the edition is better than mine - I have multiple editions rattling around my head and tend to mix stuff up.
I was trying to find an "utter low level limit" of what could reasonably be excepted to have a prayer at defeating an ancient red dragon. The reason for me wanting to do that was two-fold: 1) The OP has lots of new players, and the lower their level, the less they have to track, the easier the game, the faster their turns, the less analysis paralysis. 2) I've consistently seen PCs in groups of six or seven hit well outside their weight class when it comes to CR.
I agree that a *lot* depends on how the DM plays the monsters. The OP mentioned encountering the red dragon in its cavernous volcanic lair – which I assumed meant he wouldn't be having it fly around much, would be using the damaging lair action sparingly, and generally not playing the monster as intelligently as he would for skilled players in a campaign (since the OP is running a one-shot with several newbies).
While I was using the DMG to reach my suggestion of "about 6th level", I was actually looking more deeply at numbers and making some assumptions that I should be clear about:
- Any sane party going up against a red dragon – and knowing it in advance – is going to prepare with fire resistance at a minimum. So I'm assuming most or all of the party have that through spells, race/class, magic items, or potions. So that reduces its breath weapon from 91 fire damage to 45 damage. And happens to be just behind the hit points for a 6th-level fighter with average hp and 14 CON (10+2+(6*5)+(2*5)=52). The idea being that the DM doesn't want the dragon to wipe out a front-line PC at full health with the breath weapon.
- Similarly, I assumed the DM wouldn't focus fire on a single PC until that PC is downed & then coup de grace that PCs. While that's sound strategy, I *think* the feel he's going for is an epic battle with most of the PCs imperiled, but still not pulling out ALL the stops – because there are many beginners. So it's splitting its bite (35)/claw (17)/claw(17) attacks among multiple targets. Again, this is so it doesn't take out a front-line PC in one round. "Gentlemen's rules" to give newbie's a sporting chance in the one-shot.
- I assumed in such a large party (10 PCs) that there will be a dedicated healer with spare the dying & revivify, so PCs will be down-and-then-up alot.
- I assumed the party would accomplish some kind of alpha-striking trick to surprise the dragon and that at least one PC (likely played by a player familiar with 5e who optimized their PC for this one-shot) would open by delivering a strong blow, possibly imposing a condition as well.
- I assumed most PCs would benefit from bless or advantage or other modifiers to boost their attack rolls, and thus assumed the 10 PCs would be able to dish out somewhere around 140 damage on the first round (plus a likely alpha strike), albeit diminishing in the 2nd and subsequent rounds as the dragon kills them off.
- Lastly, I assume the PCs have ample time to prepare before entering the cave, so they can cast mage armor, mirror image, warding bond and so forth. Not sure if the paladin has anti-fear spells, but I'd also expect the cleric/healer to have calm emotions as an option to counter
the dragon's Frightening Presence.