By the rules if the PCs have sufficient cover, Stealth works against blindsight.
They dont have cover. The dragon is peeking over the top of it. Like a rogue peeking through a gap in some bushes, or over a low wall. And even if they did, blindsight trumps it. Blingsight isnt (necessarily) sight (read the ability), so blocking LOS doesnt affect it.
So it is a large pyramid. That's why my group likes things drawn out, so they understand the terrain which can sometimes be different in the DMs and players minds and affect play in a manner that leads to unhappiness and feelings of cheating by the DM. And yes, I've had to deal with this one as well, even though it was just a miscommunication.
And when they get close enough to see it, I'll happily draw it out for them.
Whether Stealth works against Blindsight is not up to the DM in a game between players and a DM that have agreed to follow the rules. Stealth works against Blindsight per Jeremy Crawford who defines the rules for D&D.
Stealth doesnt work when youre being looked at as per the errata. Blindsihgt cancels all darkness (it doesnt exist). Youre (relative to the creature with blindsight) standing out in the open.
Maybe in your games, PCs can stealth across an open field with a creature looking straight at them. Good on you. But for the purposes of this encounter, they cant.
In a situation like this we're not sitting around your table where your word is law.
Youre certainly not. Youre way too argumentative and rules lawyery and intentionally obstructionist for my table.
However the adventure was designed to be run by me as DM. As discussed above, I have enough leeway here by RAW to decide when combatants can see each other. I alone determine where the dragon is looking, and when stealth checks can be attempted. Mearls has been crystal clear on this - Hiding effectively boils down to 'ask your DM'.
Feel free to be more lenient in your own games. I suggest not being so lenient if youre having a hard time with players stealth steamrolling your encounters (which you are). By the way, its your own lenient interpretation that leads to these tactics being 'optimal and used all the time'.
In my particular group the bard has the same visual range as the dragon and access to a gem of seeing and a really high stealth herself. I think is +10 without pass without trace. Then again my group doesn't have pass without trace, so no worries. But a +20 stealth check for the bard with the gem of seeing and devilsight gives her a good chance to see creatures hidden in really dark places. I'd hope you would consider that in your rulings.
If you dont have pass without trace you dont have it. Also, does it not use concetration? How come youre suddenly not buffing the party with bless etc, walking forward on your own (and outside the paladin aura you always stay in) and using a gem of seeing.
Its almost as if youve adopted your tactics to the encounter, without knowing about the encounter.
It would be extremely difficult. I'm not going to lie. That dragon might get a vicious surprise round.
Bear in mind, youve just fought an archmage capable of casting 9th level spells with a shield golem buddy, a Githzeri war party with access to lighting bolt and wall of force, a bunch of souped up shadow demons, 2 death slaadi, and the giants and wolves. Resources are very low by this point.
A flubbed perception check by the party and a surprise round could follow. If the dragon rolls well on initiative, and follows that up with a 5 or a 6 on its breath weapon recharge, you could be hit with 2 x 16d6 necrotic (DC 19 possibly at disadvantage) for half plus a 4d8 thorn wall legendary action before acting. Buff spells down, party seriously wounded and needing to heal, and facing one very angry critter with over 400 hit points (barring being hit with radiant, force or psychic) legendary resistances and 80' flight which is capable of an additional 40' via legendary actions (while possibly being trapped in the room).
Depending on resource expenditure by the party to date, and how dire it was looking, I might be lenient with the Atropal, and waste a turn monologing a bit (taking the dodge action while I do so) and mocking the PCs.
He's a nihlist with a deep booming monotone voice.
Might also grant the PCs a perception check (DC 10) to notice the greatsword suddenly sparkle on top of the pyramid. If one goes to grab it, Black razor might meet him for a one on one fight on the pyramid (using lair actions to wall off or entangle tje other PCs - lone PC vs Atropal. Hopefully a PC rolls a nat 20 and cuts his head off trapping him. Would be epic.