I'm cherry picking?! You've reduced dragon power by a huge margin by not doing max hit points when it breathes. The fact is, the group has no idea that dragons breath anything and if it cast sleep nothing happened, then it followed it up t hat nothing with like 6 points of damage from a magic missile. The party wouldn't even know to spread out or run before the breath killed them.
What resources does the party have? Magic missile? One bow? This group is probably missing with most of its attacks and doing very little damage.
If the text of the module has to reduce a thousands of years old dragon to a newbie imbecile in order for the group to even have a chance, then they had no chance to begin with. And as for cherry picking what a dragon does, that's the DM's job! He runs the monsters, especially dragons, as they should be run, not as incompetent boobs.
Also, I pointed out why the group is going to go to the building. Having a single intact building in a large ruins is some of the strongest player bait that there is. It's like shooting fish in a barrel. They're almost always going to investigate that outlier.