I think there's some interaction between the box and the fan so that the box tries to go back to the ledge when I land on the purple wall and the box's hitbox collides with me and bats me into the sky. Probably already known, but figured I'd post it anyway.

You can get up to the railings in the roof with that. But there is no reason to use it, cause you don't do anything other then the guidedslide in floor 2 in all sigil.

Yes, that's known. Purple barriers can exhibit some strange properties. There are other ways to do similat launches, but none of them seem to be useful or consistent.