sorry for the huge surge of question but I just can't get my head around all these cases :D
firstly, why is you not in genitive case due to the negation of the verb. secondly, is tobie interchangeable with ci?
The verb "ufać" takes Dative. Now that you know that, this article should clear any doubts: https://forum.duolingo.com/comment/28545847 :) (but tl;dr - only Accusative changes the case when negated)
Not exactly. "tobie" is the emphatic form of "ci", which is neutral. So while you will simply say "Ufamy ci", the sentence here contrasts 'her' and 'you', and this very clear emphasis needs the emphatic form "tobie". Most pronouns don't have emphatic forms, but if the given one has it, then it's necessary in some contexts.