I was trying to order two dwellings at one time and the game crashed

Anyway should by able to send units to other rooms than guard rooms, but only one stack a room, except boss room, no units are allowed there.
Shift-clicking isn't an issue (I did only a little test, so maybe there could be a way to abuse that, too), because the trigger seems to detect every single click in the shift-click sequence, and sets the "real" rallypoint to the last room clicked.
It's more complicated with the doors, because if they have to work properly, they also need to be excluded from unit pathfinding (for units that could pass through them). So I thought of a system, where the doors don't have real pathing blocker. Units will pass through them all the times, they will open for them (so it looks good), and heroes will be "pushed" out by a trigger. It would be nice, but it seems like a ton of work and I can't get myself to start working on it (also, I can't decide whether to build doors on tiles, like in dungeon keeper, or on borders between tiles)
Underground passage could be done, I was thinking more like sending units from back of the dungeon to guard rooms in front, and once heroes conquer it, you collapse it, so they can't get to the dwellings, but both ways could be used ( if they should be used to divide the dungeon in 2 parts, they would have to be indestructible, so the heroes could loot the treasures in the second part). I am adding it to the "to do" list.
The problem with the spell could be that it "picks unit in range", which takes longer to find out with 1000+ units the game is using. That is the source of lags, not leaks. The problem is that each bookcase and pillar is a unit. If there are ~600 tiles + 25 units per room + whatever is spawned, it could easily get over thousand, mostly in the third round. I am thinking of changing bookcases to destructibles, which would also make them inflamable, but much less demanding on CPU.
Pillars are targeted by some AoE's, like Arcane missiles explosion (which is almost never used, it's chained effect of dispel. I once saw some magi with the axe using only blink to create lots of mirror images, which were quite powerful with the axe). Well it's true you can't collapse pillars, but I also thought of a system of indirect control of units, using double clicks. If DL double-clicks unit, he forces it to use it's special ability (would be useful once units get some abilities

), double clicking a pillar forces them to collapse it, double clicking a pipe forces lightning-casters to cast there.....
Also, you can use traps to collapse them. If you put 2 lightning traps right under a pillar and third one somewhere else, it would blow both of them out, causing the pillar to collapse. (I am adding an option to lightning charger to turn it off, so it can be used in such way)