^ you know when she looked into his eyes and said he was young, I assumed she was not looking at the age of the regeneration, but the age of his soul, that said the episode did leave me with the impression she have known him as David Tennant.
Although, there was never anything said that is proof that she was with Ten, she said he was young, but that could mean in regeneration terms.
As said, centuries couldve passed between episodes and specials. As we dont really know how the Doctors regenerations age, my guess is very slowly, it is perfectly viable for this to happen between said specials etc.
I'd prefer not to see or hear about her, my imagination has taken that role, and its a good one. Your outcome maybe different to mine, but Moffat used the best device ever to close the episode, the imagination.
Besides, Moffat being the all clever little thing he is, he gave us one line, ONE line only to explain why we never saw inside the book, never heard the truth behind River and The Doctor, never heard anything really explained...
ONE line explains the whole thing, the whole reason why we will never see River Song again or an explanation to the who fiasco.
ONE line that us fans can relate to, a line that gets out imaginations going in overdrive...
"Spoilers, Darling."
Sums it up beautifully for me.