I agree about the whole Pella Henry thing being very odd, but I guess it was a symptom of Henry's loss of self and Pella's apathy. And maybe it needed to happen for the final redemption? Pella & Schwartz who were obviously made for each other needed a break too, to make the ending better. At least there was a proper ending, and kinda nice that Henry took over Schwartz role.
Have you read A Prayer for Owen Meany? It reminded me of that while I was reading it, particularly the two central characters. But I also read The Marriage Plot just after The Art of Fielding, and now looking back am getting the two a bit mixed up, though at the time much preferred The Art of Fielding, far more interesting characters.