I might not sound like it, but my head canon is actually pretty loose too..
My head canon for the warden was... travelling the World. (That was what she said she would at the end of da:O). So the whole calling plotline doesn't really chang ethat, but that because we only hear about it.
My head canon for Hawke was hiding with Anders and properly doing something for mage freedom, which was also pretty adaptable.
The problem is not what they make the PC do. I can adapt my head canons to changing circumstances. It is with their personsality or the motives they ascribe to the PC.
Had Hawke said nothing about blood maigc or simply said "Varric told me you had questions about Corypheus" then I could have adapted my head canon easily.
I would have just gone, so:
All right Hawke is very pro-mage and Quizzy helped the mages with an alliance, she doesn't trust the chantry, but Varric properly wrote only good thing about the Inqusitor and Corypheus mind controlled Anders once and her brother is a warden too, so she would be worried. Thus Hawke turned up at the Inqusition.
But I can't adapt the head canon, because bioware suddenly decide personality, morality and motives that Hawke just never had. In the Keep there is one, one, opinion tile and that is 'What did Hawke think about Anders' action'.
If they want to bring PC back as NPC they need a lot more morality, personality and opinion tiles, to just have a chance of getting the Pc's personality right. And while it could be awesome, realistically I think it is too much work and so I think that it would be better to just keep the former PC to their own devices.
Also I would like to repeat. The Pc's have been through a lot by the time their game is over. They deserve a break.