1) no u don't need for just 2 weeks but if u are planning more than 21 days u need visa already
2)yes she need visa but if fiancee visa,u need documents as per your #6 words..keep it
3)thats another proof of your real relation when u visit her in PI
4)as far as i know,yes she can.. like wise if an English man currently on divorce process he can apply on fiancee visa right? u can correct me on that if i'm wrong
5)if u will get married here,she can sort it out as far as PI is concerned for sure and keep posting so u will able to search forum because there is lot of additional info u can have here....its like gem lol
6)no..that wasn't problem but don't get marry there..as more complicated
just marry here in PI
7) Christina can petition his son once she will be uk citizen after some years

wait for another members reply on your post as they are more and well experienced than me

good luck and welcome to the forum !!!