Sadly the undo button seems pretty useless when it comes to terraining. As far as I know there's no other ways to undo mistakes. Just one more problem with the editor that hopefully they will address, though maybe their system doesn't play well with undoing stuff for some reason. I mean, a functioning undo is pretty up there in importance for an editor of any kind in my opinion, so there may be a reason why it's not as good as it should be. Or maybe they just got used to the editor and learned how to not make mistakes all the time. They did post some shortcuts in the "Creating Combat" document regarding playtesting stuff (like CTRL+SHIFT+K to kill an enemy I believe), but not sure where else there's a list of more general shortcuts.