If one was accidentally to use 'вона' here when speaking, would it be a serious deal or just an odd look?
It would be just grammatically wrong and no more. That is, you cannot insult a person with this or something like that.
I've used "її" instead of "їй" are these datives (not) interchangeable ?
Її is the possesive form of her
So вона and їй can be freely interchanged or a set of rules exist?!!!