AA = rubbish hand?

I think you should call. You are getting a good price, and he has lots of semi-bluffs. If he doesn't have Kx, he could conceivably give up on the river.

Plausible. I'm just struggling to imagine that many bluffs, or that he would barrel to rep the K, but that might be reg-brain on my part

Fish do bluff, but IMO they don't bluff enough when they show aggression on 2 streets for big sizings when their UTG RFI opponents AK is a strong hand. A9cc would be a better bluffcatcher, but not happy either way.

SB preflop call and shorter stack would make me just call down.

We know from MDA that fish overbluff XR-B-B by a lot.

Call again and plan on calling river.

Any straight draws, spades, worse pairs, random cards. Plus you hit the nuts 5% of the time and have good implied odds.

I think to a reg brain the turn is a bad card but to a fish brain it doesn't matter. So their range is probably far wider than might be instinctively accounted for.

i.e. if the plan was to make mr cbet guy pay then it makes sense to a fish to barrel the K big, as if you have a King/set. Whereas a reg is thinking 'wtf u polarising bro?'

Actually SB should be polarising on the turn with at least an 1.25x overbet:

So if the mistake they're making is giving too cheap a price then it seems logical these so-so calls are more compelling.

I've seen this play a lot I agree it's one of the most common bluffed lines in micros. I think the thinking is that a second k coming to villain makes them think it fits their story even more that they were raising with a k on flop.