Re: the Shadow demons, I'm not seeing it. The PC's can simply ready actions to [attack/ cast a a spell] triggered to occur when the Demons emerge from hiding [or the floor].
Taking 1d10 damage no matter what is certainly going to add up, but I still think it would be worthwhile. Readying an attack will eliminate a great many benefits that martial characters get (uses action + reaction, no extra attacks, barbarian can't Reckless Attack, fighter loses its second extra attack, monk can't FoB) and seriously hampers spellcasters (must concentrate on readied spell, spell lost if not used or concentration broken before used).
Even if there's only a single fighter, going from 3x 1d8+5 (28) damage from the Attack Action with two Extra Attacks using a longsword down to 1d10+1d8+5 (15) damage is a MASSIVE decrease in damage taken, with the additional benefits of screwing with the party's action economy and providing a way to escape if the party ends up being too strong. For each extra member of the party the benefits of hiding in the walls increases. It also limits the party's ability to focus fire on the shadow demons, preventing them from losing combat effectiveness as the fight goes on. Lastly, if we consider a demon hidden in the shadows, then we must realize that it will become immediately visible as soon as a character carrying a light source gets close enough to it for the demon to be in bright light (20 feet for Light or a torch). This will entirely eliminate the chance for surprise AoO from running by a hidden demon, and expose the demon to the focused might of the entire party.
Since the shadow demons are hasted, they have a fly speed of 60 ft, cut in half from moving through walls. If there is another room within 30 feet that is out of LoS of the party then they can entirely ignore the 1d10 damage from ending in an object. I'm not sure how dense the complex is, but it's not unreasonable for there to be somewhere nearby. If there is a floor below or above then this becomes trivial
I think that trying to hide in the shadows would be the first plan that they would use, but after they are found despite hiding (either from bright light, truesight, or high perception) it seems reasonable that they would use their incorporeal nature despite its minor downside.
My own opinion on the most effective way to fight Shadow Demons is to exploit their Light Sensitivity (not sunlight sensitivity). Someone casts Light, everyone grabs a torch, and now the Shadow Demons are all attacking at disadvantage (less likely to hit AND incapable of dealing their boosted damage from attacking with adv.) and unable to hide while near the party. They will be visible to PCs with darkvision while within areas of dim light, making them immediately visible. They will also be visible if they rise from the floor into an area of bright light.
I'm not sure what the party's goal is, but I assume it is to get past the room somehow. I basing this on the complete lack of benefit to actually destroying the demons and the fact that it is explicitly stated that they will not pursue the party out of the room. If the party is aware of this goal going into the fight, then this encounter is actually simplified if the demons try to hide in the floor or Hide in the shadows. Skirmishing types who rely on avoiding detection as a defense tend not to make the best barriers to party progression unless there is risk of being flanked if the enemies are not eliminated. Of course, it's perfectly fine to let the players assume that the demons need to be destroyed by being ambiguous with the encounter goals (not my preference, but it can be used effectively), but then we're back into trying to optimize for victory, and that still means not being exposed off-turn.
I want to note that I am not replying because I think your design is bad. I am replying because I am interested in your experiment and its result, and because I wanted to present an alternative perspective along with the reasoning behind it. I personally think that making incorporeal creatures perfectly safe between turns vastly increases their strength, and that it should increase their CR dramatically when used in that way. However, since you are trying to drain party resources as much as possible, and because it can increase their effectiveness, I felt it contributed to the thread to present it as an alternative tactic.
Edit: It just occurred to me that not being able to see the map and not knowing the party's goal make it so that I cannot guess what purpose the shadow demons are supposed to serve. If, for example, environmental effects make it harder for the party to produce light (I've used this in a game before), or if the demons' goal is to inflict as much damage as possible before being destroyed, then my proposed tactics are almost certainly not a good match for the encounter.