You know, I hadn't thought she'd seen quite so much, but I have been thinking about the fact that the Doctor, once again, has made an assumption and assumed that Rose made Jack immortal by mistake. I'm still leaning towards the notion that she totally did it on purpose, for many of the reasons you laid out, and the reason she forgot everything afterwards was because she saw too much and couldn't know all that and react naturally. And if she remembered anything of what happened (as in Doomsday), it was very vague and mostly based on what the Doctor told her.

She may have even laid out a safe path for her to travel back from the AU, which is what we see her using in PiC.

*bounces* Oh, I hope this is even vaguely correct.