If there are, say, 5 characters in play, I reveal Threat, and their STR is reduced, can Meereenese Brothel respond to each of those STR reductions?

Sure. Was there a specific thing making you think otherwise, or was it a "seems too good to be true" thing?

You would, however, need to be able to stand the Brothel as a response in between responding to each character. Otherwise, at the point when the plot is revealed, you will only be able to trigger it once.

It was a more too good to be true thing, considering I totally overlooked the kneeling requirement. Should try reading my cards sometime. Not finding any way to abuse that as it is. Thanks anyway, it solved a timing debate we were having.

I found out last week (if you have a deck with GJ and targ in it) that Ancient Mariner triggers the brothel too. He doesn't even need to be part of the challenge either.

Is this true? He says "-3 STR during MIL challenges". That includes even when hes not participating, it seems.

It is true. It is known (added to make the post long enough )