ClaytonCross
Kinder reader Inflection wanted
A few notes on thst dragon fight... you should expect fire resistance at that level. Most parties are going to find ways to get resistances st thst level, especially fire.
Could be spells, abilities, magic items, nature domain, whatever...but a party of thst level going against s real dragon will find a way to get resistance.
Also that warlocks hitpoints are way too low. 100 hp is going to be your norm, either by base or some temp hp ability. To put it in context, every single one of my players at 20th level had over 200 hp...but thst was with feats and an extra +2 con. Our barb had over 300. Oh and thst was before inspiring leaders temp hp.
Lastly the warlock should have +12 minimum....and probably advantage. Again at those levels you are going to have something that boosts attacks, could be a magic weapon could be bless. And there are many ways to get advantage at those levels.
And I think that right there is where high level crs break down. There are certain bonuses at those levels that you just have to expect. You may not know how the party gets them...but you have to trust thst at those levels they just will.
I got my stats strait from the players hand book AFTER asking for the actual players stats and not getting them. So your estimates are just as arbitrary as mine but I had to work with something since he wanted to be vague. A 10 constitution for a warlock is not bad an I gave him about the best armor he could have as a warlock. I could have given him more hp and more average armor which would have raised all the DPR and possibly off set the HP. I feel I was generous with an AC18 warlock so I don't feel then need to re-run the number at least unless actual stats are provided.
As far as fire resistance and proper planing it comes down to if they know or if it was a surprise fight. I am going with surprise because I think there is no accounting for good preparations. I mean if I was a fire dragon with fire immunity and knew a party was coming for me. I would turn on an anti-magic field, flood the place with a foot of lava after they entered, and watch them burn to death. So I am not really going to consider preparations.