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Ass.1) All 8 mirrors must be brought over to the east side of the room.
Ass.2) I'm guessing that 4 trips are necessary, carrying two mirrors each trip. (Otherwise you would need to juggle three mirrors, on two separate trips)
Ass.3) There are a few firetraps that are toggled by more than one plate, but this doesn't seem especially relevant to solving the room, as those firetraps are mostly irrelevant until the final pass.
Looks like the fourth pass runs out of trapdoors and is forced to travel over a bad plate.

×You need the trapdoor south of the vertical 3-plate for the last pass as well as the eastern-most trapdoor.

×Not putting all the mirrors on plates right away, as plenty of the doors are far enough away that the tar won't reach them before you complete your next trip.