The TARDIS powers down and goes dead. It's scary. They hear the birds again, and remind each other to remember how it feels, that it feels real on the TARDIS, that the other life is just a myth. The Doctor seems to say this a lot to old Amy, doesn't he? Begging her to remember things she doesn't know yet, and things she never knew. Tears in his eyes. Asking him to trust her when the whole world says he's wrong. Keep those eyes shut, but act like you can see. Hold onto impossible things. Grow up without growing hard. Stay young without staying hard. I think this is the key:
The only way he can help her -- to make Amelia and Amy be the same person, and make that person whole -- is to keep pushing at her from both sides. It's not about Rory or weddings, those are just the clothes this story wears. It's about a God who fucked up, and has to fix what he made. Who has let two moments in space and time touch, and cracked the world. And if you've ever been a person, you know how much easier it is to change, to grow, when you've got things pressing in. When you're trapped and you can't get out, your only chance -- the only choice you have -- is to turn into something else. Seen from this angle, even popping out of Rory's cake like a stripper was necessary for the Doctor to gather them together and push them through this mess he's made.
Amy's already made her choice. She chose something impossible, which is to grow up and marry Rory -- but stave off that moment forever, or as long as she could, by traveling out of her timestream just before anything happened that mattered. When you are forever, you are also never. The moment never arrives, like with a black hole. So by entering Time, L'Engle's kairos, she doesn't have to experience time, chronos. Amelia was the one that made that choice, and Amy's been paying for it ever since, but it's the Doctor's fault.
Usually when Companions do this, and they always do, they're changed by the adventures themselves, but in Amy's case, her adventure started with Amelia, so that's where it has to end: Not on her wedding day, but in her own repair. Bringing Rory along to Venice could then be the Doctor's way of forcing her to include Rory in her calculations -- to bring him into the trap with her, to help the Doctor save her -- instead of thinking she could leave him on the slow path forever. And all things being equal -- that is, leaving out the totally gross gender iniquities that are all over the place, just like in most stories -- that's pretty awesome.