timeline - Why didn't the Doctor know he would regenerate again?

"But Clara should have remembered Twelve's appearance, since she was presumably with Eleven in his Tardis and no older version of herself was around"

As you say, it's also not clear that Eleven actually "summoned" him--it's possible Eleven just sent a signal summoning all his past selves (and telling the First Doctor to begin the necessary calculations, since the Gallifreyan General said "The calculations alone would take hundreds of years" and Ten said "Don't worry, I started a very long time ago" to which Eleven added "You might say I've been doing this all my lives", and then we see all the previous Doctors arrive). So, Twelve definitely could have come of his own accord because he remembered they needed all the help they could get. You say that Eleven would have known anyway because the Gallifreyan General said "Thirteen!", but it's not clear they were still transmitting audio to the Doctor when he said that, and anyway all the versions of the Doctor looked pretty distracted by the task at hand at that point.