1. Kristen would absolutely be welcome given that she has both a backstory and a personality. No problems there!

2. Ivy's age-up would be handled with a canon update via the Dreaming in which she'd come back older and with all her new canon memories. The only stipulation is that she isn't aged up to the point that she'd be unrecognizable to other characters. Switching to the icons of her new actress would also be fine!