^There was also a Big Finish audio that said that the reason why the 2nd Romana looked like Princess Astra was because, when the Key to Time was split up again, the essence of the 6th piece was transferred from Princess Astra to Romana.
Lake Silencio shows nothing of regeneration. It's a time travelling robot from the future filled with miniature people. It's a trick. A light show.
Didn't that event happen two different ways because River rewrote the fixed point? So basically, the "real" Doctor did die like he was supposed to but then there was the Teselecta Doctor.
Lake Silencio is a really weird predestination paradox. It's made even weirder by the fact that the only reason why the Doctor or River even went there in the first place is because they were told that this was where & when this was going to happen and had already happened. (As a result, the Silence should have already known that it wasn't going to work because it had already happened and yet the Doctor had already made it to Trenzalore anyway.)
But, even then, I suppose it's possible that the Doctor might have had enough energy left to attempt a regeneration, probably just not enough for a successful one. (To see what an unsuccessful regeneration looks like, SEE crispy Master in "The Deadly Assassin" or the War Chief in the New Adventures novel "Timewyrm: Exodus.") I figure, Time Lords normally have exactly enough energy to manage 12 regenerations. But since Tennant's regeneration in "Journey's End" was not a full one, he still had some energy left over. And possibly a little extra leftover that he got from River in "Let's Kill Hitler." Later on, he resigned himself to the fact that, while he had some energy left, it wasn't enough to be able to do much with. Which is why he was so cavalier about using some energy to heal River's broken wrist in "The Angels Take Manhattan."