"Tell them, for example, that there were black people living in the UK in the Middle Ages; or that prisons are not 5 star hotels, and they'll claim you're being politically correct. They make no attempt to analyse whether political correctness is even relevant to whatever they're moaning about."
Exactly right and the people would be right because political correctness is all about spin for the purpose of politics and to silent the people's dissent. There is a reason that the political class reads scripted texts and why Blairites had to consult their pagers to see what the correct party line (the politically correct line) was before they spoke for the party, the bigwigs, the Establishment. There are facts, but there are also other facts which are ignored by the politically correct facts in orser to silence the people. Political correctness is spin in the service of the political class for the benefit of the Establishment and what they want the people to accept is correct. They don't want Trump's wall because they want open borders so they have to silence the people who do not think correct politically.
'They make no attempt to analyse whether political correctness is even relevant to whatever they're moaning about.'
It is nearly always relevant since it is spin to manipulate the people and the Establishment rules by media manipulation and spin as the few control the many by setting the parameters of what the many are allowed to think politically. The few deteremine what is politically correct or correct politically.
'It leads to the sort of lazy thinking that seriously believes in the Winterval myth'
It is not a myth. It is a symptom that the people see and understand. It is not very widespread, but the people instinctively understand the ruling class and know that if the metropolitan elite get theri way, then it will one day be widespread. That is why Trump gets tremendous cheers when he says "when I get in, we're going to be saying Merry Christmas, believe me folks". It is not at the top of the people's priorities but it is a symptom of what the ruling class have done to the people.
'It's a knee-jerk way of dismissing something you're uncomfortable with when you can't come up with a decent reason for your discomfort.'
No, it is an instictive understanding that the people have been spun and tricked and lied to. That is why Trump is so popular because he is the only politician to challenge it directly. Hillary was asked on an interview "have you ever lied to the American people?". She said "I've done by best not to". Greta asked Trump that on Fox last night. He said "No, i don't lie. If anything, I tell too much of teh truth, that's why I get in trouble". He meant that he is not politically correct and will say what he really believes and to hell with the Establishment and what they have said is politically correct.
'Dear me, that must mean that Obama is politically incorrect, because he's head of a government which maintains its illegality.'
No it doesn't. Under Obama there are sanctuary cities etc and the borders are open and there is no wall. It is not because Obama has decided that, it has been decided way above his head by the Establishment and the reason they want to stop Trump is because he refuses to go along with it and wants to build the wall. The government can't say "we are having completely open borders" because the people would vot them out, so they have to do it slowly. John McDonnell says "it is inevitable that we will have open borders by the end of this century". Of course it isn't but it is inevitable that the Establishment will want that.
'All Trump's political opponents are politically incorrect because they don't indicate any intention of legalising all immigration'
No they are not. They are all Establishment but very good clever actors. What they say is not what they do which is why the people have abandoned them and switched to Trump. Jeb said immigrants come to America as "an act of love" and Cruz and Rubio both run ads against each other saying that they lied about not wanting to legalise illegal immigrants and push for a path to full citizenship etc.
'Oh dear, where does that leave Trump?'
It leaves him as the frontrunner in the Republican pack, tearing his way across the USA, filling out stadia with the largest crowds of any politician and scaring the bejeezus out of the elite.