gran_orco said:

There will be a related errata question probably sent. The actual question does not in any way require FAQing and asking for it is a waste of precious FFG resources. The rules are very clear and very specific.

I have prepared a question about errata-ing the existing FAQ answer to ask if this answer should be edited to say "...interrupt the OL at any time during his (the OL's) turn..."
If it is denied during back-and-forth then I will prepare another question directly about the guard-mimic issue.

This is just to eliminate the round-about loophole that poor rules-lawyers think can get them a wedge into an open and shut case.

OFFICIAL ANSWER FROM KEVIN WILSON:

Hi David,

I sent you an answer a week or two ago, but I guess it didn't reach you. Guard cannot be used against a mimic until the overlord's turn, so it cannot immediately attack.

-Kevin