1) No, you can't reverse it in Steam. You'd have to use the non-Steam version of Warband.
If others see this before you log onto Steam, probably best to disable auto-updates immediately or log-on in offline mode then disable them for Warband because:
2) No idea if it will break Floris.
3) Yes, of course Floris will be updated--but we need the module system to do that (and, depending on what changed, a goodly amount of time) and the module system isn't out yet.