Lol to that.
As to the ceilings, it sounds like the problem here is akin to the one sided walls. There are more complex gameplay issues though. I am sure players are going to want seperate materials for a ceiling/floor. if you have a granit floor and a gold ceiling, and you remove the granit floor what happens? Just getting a gold floor seems silly, sine at the point the exsisting adjacent floor *should* be uneven. I think the problem is that floors have no thickness, so there is no room for a ceiling in there.
The 2 ways i can see it working now are 1. Removing the floor or ceiling generates the mats contained in both and an open space or 2. removing the floor leves an unsmoothable, un removable rought floor of the ceiling material type of the level below, with the reverse true for ceiling removal.
As to the building acces argument, there are ceiling types that are built (or can be built) from the level below, such as various forms of cloth roofs, thatch roofs, simple wood slat ceilings, and of couse any decorations that go on a ceiling have do be done from below.
I like the idea of having the view toggle between floor and ceiling. That seems to be a workable idea.
The ventaltoin thing has been discussed elsewhere. You should go and res that thread, as many of the points being discussed here were already talked about there.