'You are nuts you know claig '
That's what the Establishment call anyone who disagrees with them. They call Trump a lot worse. But we both wear the badge with pride because we know we have got them on the run! 
'I've been sceptical about whether Trump had any chance of the nomination'
I haven't because I understand what the people feel and know that Trump is by far the most popular candidate. However, that doesn't mean that Trump will get the nomination because there is utter fear of Trump in Establishment circles and their puppets have been instructed to stop him and dirty tricks will probably be used against him. We the people hope he beats them.
'My impression is that anyone who has any intention of voting for Trump is already in his camp'
No there are far more to come. Trump hasn't even started yet. This is going to be a landslide.
'so he won't pick up many votes from Carson/ Fiorina/ Christie supporters'
Carson is a very good, decent honourable man. His voters are decent people and they will either go for Cruz or Trump (both outsiders and anti-establishment). Cruz has more Christian credentials than Trump, but in my opinion Cruz is really a phoney and will end up as an Establishment puppet, so I think that Carson's vote will go to the candidate whom the elite fear - Donald J Trump.
Fiorina is excellent, she is very impressive, a real fighter and not politically correct, a real breath of fresh air. I don't agree with her on foreign policy, I agree with Trump, but she doesn't like Trump much, so I am not sure where her voters will go, probably they will go to a more Establishment candidate, which in my view would be potential puppet Cruz.
Christie is Establishment and pretty liberal so he is not in the "real conservative" camp for example he is misguided on a real conservative issue i.e. global warming
Chris Christie admitted that he thinks climate change is real and that humans contribute to it at an event in New Hampshire
This sort of stuff is Establishment and doesn't go down well with real conservatives. Voters fear sell-out and puppet tendencies in this stance. So on that basis, Christie's voters are more likely to go for Kasich, Bush or Rubio. However, Christie is a brilliant fighter who can go for it if he wants to (similar to how Trump does) and therefore some of his supporters may even prefer Trump's fighting style to the politically correct Establishment puppets.
'Whether he gets the nomination may come down to whether the republicans can eventually unite behind another candidate and it's not clear that they can.'
This is what the Establishment are hoping for but in reality none of their candidates is a match for Trump and the people. He will destroy the lot of them because he is not politically correct.
'Sanders objectively has a much bigger mountain to climb, but who'd have thought he'd ever get this far?'
Yes, it shows the discontent with the Establishment and the Clinton Wall Street backed elite same old same old. But Clinton is supposed to have a firewall in Southern States among African American voters and it is thought that Bernie mainly appeals to white college students and white middle classes. But I am not so sure, let's wait and see, Bernie may surprise the Establishment and get much wider support.
'On the global warming issue, I doubt Trump is anything like as fervent a denier as other candidates - he's just an opportunist and I kind of doubt he's given the issue any serious thought, mainly because there is no mileage in it for him. '
No, the Establishment candidates are the opportunists, they will say whatever their elite masters tell them to say in order to keep the special interest donor money flowing. If Trump were an opportunist, he would do as the elite say and say "you know what, the elite are right, global warming is real and I'm going to join the puppets of the world in their hands aloft global warming kumbaya at the next Paris COP". But he says teh opposite, he says what the people say "this global warming BULLSHIT has got to stop".
'Most of the other republicans are also climate change deniers'
They say they are but no one believes them which is why everyone is voting for Trump. The other republicans are not as modernising as our so-called conservative politicians, but all the voters know that if they get into power, they will do exactly what the elite tell them and be "modernisers" too.
'It would be interesting to see Trump dare to buck the establishment on this issue as it would give more credence to his claim that he can't be bought (but I'm not holding my breath).'
You are exactly right. The issue of global warming is the real litmus test of a true anti-establishment, populist, honest, truthful candidate. That is why Trump will never let us down because he tells the truth and no amount of elite money can make him lie. Trump doesn't tell the public the full truth on everything because he knows it would shock them, but the elite knows he knows the truth and that is why they fear him. Trump will always tell the truth on global warming because the elite don't frighten him, can't buy him and can't bully him. That is why the American Republicans are all in for Trump 2016!