About the hallway: it was about ten feet long when Cho climbed up, and about five feet wide in both directions, so about the size of a phone booth or small closet?
Cho and Ebony are rather snug in there. There's not a ton of room to maneuver. Cho placed the decay at about the six foot mark, and it spread up and down about two feet in either direction before he shut it off. Right now, He and Ebony are at the lower end o the hallway, Cho's lower claws dug into the sides less than a foot from the opening. the hallway is bowing in at the midpoint, and the lower half is constricting (currently down to about a three and a half foot diameter - enough room for you two vertically, but it's quite intimate), but the upper half seems to be remaining open. Active buffs are Ironskin and I assume you didn't deactivate strength, considering the sudden change of situation.
An opening did not appear in the hallway when you rusted it, but it looked like decay would do the work, given a little more time.
Ebony is practically coated in that moss, and it is clearly and visibly affecting her. Leaving her behind would quite possibly be a death sentence, even if you are planing on coming back for her quickly. You can't say for sure, not really knowing Ebony's spell list or if she is resourceful; enough to save herself, but it looks pretty grim for her at the moment, while Cho has avoided the worst of the effects and seems more capable of continuing to do so.
I think that covers everything.