If you make structural changes to your filesystem without a journal, and you fail mid way, there is a 100% chance your filesystem is not in a known state, and a very good chance it is in a non-self-consistent state that will lead to some interesting surprises down the line.
FAT has two allocation tables, the main one and a backup. So if you shut it off while manipulating the first one you have the backup. You are expected to run a filesystem check after a power failure.
No, it is very well known what will happen: you can get lost cluster chains, which are easily cleaned up. As long as the order of writes is known, there is no problem.