I think your second sentence is on the wrong track, and I'm not sure about the third, depending on what you're doing when. Your last statement is correct, and is the first thing you should do. I'll put what should go on what pressure plates in secret tags; that might help with unpacking the room.
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×The three roaches activate the three western pressure plates. The guard holds down the northern on/off plate, and the final southeastern plate. Beethro steps on the plate opening the door to the trapdoor.
And a note on blocking guard pathmapping:
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×Guards (and brains in general) don't pathmap over arrows.
It's a neat room, but the guard manipulation's pretty tricky.
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