'he certainly doesn't have the same opinion now as he did then. So, which one is his actual opinion? '
His current opinion because he has learned more about what really went on in the meantime. Trump is not an ideologue, he changes his mind as he finds out more information. He will change again over time if he learns new information on what has really been going on and who was really behind it.
'And if he was so wrong back at the time, can we trust his judgement when it's time to act? '
But he wasn't so wrong. He was against the Iraq War since it started, when the politically correct Blair and the gang were all for it. I trust his judgement but there are others who prefer Clinton and Establishment Rubio.
'Even if he wasn't a politician then, he must have kept up with the news, if he was contributing his opinion. '
He did which is why unlike the rest of the Establishment, Blair and that type, he was against the Iraq War, he wants the 28 redacted pages of the 911 report opened up etc
'I'm not a politician and I did predict when Iraq was invaded that it would end up a big mess with a guerrilla type situation (as it has been), regardless of the rights or wrongs of invading it then'
So did Trump because he was also not a politician and did not stand to gain from it.
'So, can anyone trust his judgement (without the benefit of hindsight)?
And can anyone trust he believes anything he's saying now?'
That is up to the American people in the election. But it is obvious that the Establishment, the Economist, Davos and all the rest of them believe he means what he is saying now which is why they are all united against him.