but now I'm wondering if Ianto was actually quite outgoing and cheerful between the fragments flashback and cyberwoman, and only changed after Lisa died.

I think you might be right, about Ianto changing after Cyberwoman, it's just that he got so little screentime before that his personality was just assumed to be quiet, reserved, and held-in. But yeah, there's that scene in 1.02, and in 1.01 he's definitely got the sass when he makes the harassment comment. I can't remember him at all in 1.03 and then...well, i think we all know what happens in 1.04.