The answer is: YES! we do intend to allow for the editing of finished buildings. We haven’t done so yet because it’s hard. T__T. Aside from pathing issues to whatever location you want hollowed out, the building, as I understand it, is saved all together, as one structure, for use in RPG mode. Allowing for the editing/merging/partial-destruction of buildings means the building envelopes etc need to be edited and merged too.
I don’t think we intend to allow reversing of mining regions, because IRL, you can’t really undo what you’ve dug out. But you could fill in a mine with blocks, if you wanted.