I know very little too, so Ill try to chime in with what I see..
Does it go through to the other side? It may not be a crack but a poor folding weld, which is not a serious problem. If it goes through to the other side or penetrates into the hardened edge, its a definite value killer.
This implies to me, along with the shape of the base of the tang, that the blade has been remounted at least once, and possibly shortened from a longer katana/wak. Cannot tell without seeing the hamon at the neck of the tang though.
Probably not. The tsuba is required to be in place on a sword to ensure everything fits together tightly, like a puzzle kind of. The tsuba was probably removed at some point for storage or who knows, and the leather 'washers' were probably made and put on just to keep everything in place with it off.