All this is over-stated, I think. This time around, I'm much more drawn in by present-day Quentin, pre-Parallel-Time. Having him be the cad of 1897 would not have been interesting. That would have been throwing Q back in where he doesn't belong. 1969 Quentin was more worldly, more mature, and more empathetic from all those years of experience. He's very centered and grounded "now", and while that doesn't scream out excitement, it's great storytelling and acting. It's subtle. It would be great storytelling if they'd pursued it and fleshed it out further, that is.
Up to the start of PT 1970, everything about Quentin does work and make sense. Of course he has a perfectly good reason for showing up at Collinwood shortly after Barnabas returens from 1897. Quentin has known this would happen at around this time for 70 years. Would you be able to stay away? How could you not witness this trick of time for yourself, and not greet the time traveller on his return home?
The amnesia creates some mystery which unfortunately isn't totally resolved, but it's not embarrassing. Once he regains his memory, of course he's going to help Barnajulia. They're all too busy Leviathan-fighting to build up a life or recent backstory for Quentin. He doesn't seem to be living at Collinwood pre-PT. Still living out of a suitcase at the Inn.
Post-PT, I agree with what everyone's said.