Okay, and to make sure I understand the bit about the left edge or right edge:

Assume that the first level had eight rooms labelled A-H, it had stairs going down from 1B and 1G, and the second level ended up with two disconnected sections explored: A-C and F-H. Would A, C, F, and H all be permissible for removal as long as they aren't occupied by players? (Note that allowing just A or H might lead to a problem once those two are gone.)