Good idea. My fear -- not knowing how it works under the hood -- is that the diplomatic response and the Guard might not be lock-step sync'd and that would be bad: there shouldn't be any exploitable gap between the ally fleet releasing Guard and your fleet Guarding.  Granted, any mechanism that causes them to release and not immediately re-Guard is a step in the right direction, even if a third ally could jump in and sieze it on a multi-party assault.

Yap. In my last game, a "helpful" Vodyani ally indefintely parked two scout ships over two pirate bases close to me, which then regularly spawned ships that destroyed my arks, and I could do little about it. I honestly cannot tell to which degree that was WAD, but either way, lots of fun :-/