Probably a silly question since we're going to house-rule it...
Should KET read Action: Return KET to "his owners hand" rather than "your hand" - if I play Stand and Fight on a Took in someone else's discard pile and trigger his ability to return him to my hand, I won't be able to play him again if I don't have Leadership heroes.

Correct. It is placed "under the control" of the person who casts stand and fight. Triggering his ability will send it to the tnd and fight casters hand, not the original card owners hand.

There's a longstanding convention in all CCGs and LCGs that it's not possible to place a card that you do not own into one of your out-of-play states. I'd expect to see this errata'ed soon.

radiskull said:

Some of the CoC cards also use this wrong wording, while others are worded correctly.