Your mob room will eventually fill up with spiders unless you make a grid on the floor .... since mobs don't spawn on stone half-blocks... cap every other row with half-blocks. Now you'll only spawn skellies, zombies and creepers! Yay!
Yes, the space is 2x wide so spiders will fall into it for thread.
with the bottom floor being the lava/water trap and the 2nd floor being the water channels to carry the mobs to the center.
My problem is that the mobs hardly (note: a few do, but rarely) wander over the edge, and I think a part of this problem is that the lava from the trap is visible from the center of the room, from 3rd floor all the way up.... could this be the issue?