He said criticize, not insult.
Splitting hairs.

but saying Khan shouldn't criticize them is r e t a r d e d.
He's not critisising his policy, he's saying stupid shit like he's become a "poster boy for the far right", which is nothing more than a political pot shot. There's no substance, just rhetoric that is designed to appeal to his leftie supporters.

Critisising policy is fine. I won't argue with that. But political jibes are insults, not critisisms.

Even if the woman who asked the question nodded (she didn't say yes)
You're right, but I have to assume she nodded, because he went on to discuss Khan. But even if she didn't nod, she didn't clarify her question.

I wouldn't fault her for not seeing this as a deliberate misdirection but rather that he might have misheard.
I would. She should clarify her question. "No, Jeremy Corbyn. Can you work with him regarding trade?".

But of course he didn't mishear
This is your assumption, and is subjective.

it's clear as day that she was asking about Corbin
Not at all, the woman literally asked May straight after if she though Khan was a "stone cold loser". So it's completely understandable why Trump thought that was who the question referred to. Again, this is your subjective opinion (and mine), not an objective fact.

Anyone not fucking retarded knows that that Khan wasn't the subject of the question.
Subjective.

HE WAS SPECIFICALLY ASKED ABOUT TRADE!
Didn't he quickly move on to specifically discuss trade?

He chose to answer a different question. Why do you think that is? I have a pretty solid theory over here if you want to take a gander!
Let me guess... it's your subjective theory not based on objective fact? Fire away.