About the Tesselecta and regenerating. I had assumed that the Tesselecta was faking the regeneration, because we've seen it forming a motorcycle for itself and shooting red and blue hell energy from its mouth. But if they jumped the camera around to not show it, it's possible that the arc was changed. But if it was a fixed point, I'm not sure if he would be allowed to change it. We saw what happened when River tried.

The frozen reality must have formed the first time too. It must always form at that point in time. But we didn't see it when we were getting Amy's point of view. So the only strange thing is why Amy was remembering things from the frozen reality this time, but not the first time. I'm not sure I can think of an explanation for that. The only one that works is that the Amy, Rory, and River in the frozen reality were not the ones from the beach, but instead were the ones from later in the series after the Doctor had left them, and so the memories from the frozen reality didn't enter their head until later. For some reason.

I thought the Doctor-Tesselecta showed a lot more range of movement and facial expression than we saw out of the robot in Let's Kill Hitler, but I guess I'll explain that away by the Doctor being really good at piloting it, maybe using his knowledge to improve its technology or something too.

It was a good way to tie it all together though, or at least as good as any. If it had been a ganger, you'd have to deal with the fact that the Doctor had let someone die for him, which is something he wouldn't want to do. It answered how he could survive yet still be there for the fixed point. It also answered the question of where his TARDIS was during the whole episode. It was inside him.

Also, the Doctor was being pretty careful and subtle about the whole thing. He really didn't want anyone to suspect. He didn't even want River to know. He even made the Tesselecta "grow" its hair out and "grow" a beard in the frozen reality.