The simplest way I can put it: "To you" implies two things: 1. Something that you already have/know. 2. An action towards you. Example: I gave that apple to you as a gift. I am running to you. "For you" means something that is about to be given or something was done. Example: I left dinner in the refrigerator for you. I took the laundry out of the dryer for you.

It's much more likely that I would say "if it is convenient for you". I might be making a specific, concrete suggestion for an appointment, for example: If it is convenient for you, we could meet tomorrow at 2 p.m. If I said "convenient to you", I would probably be referring to something more general or more theoretical -- i.e. something less specifically connected directly to 'you'. I might be asking someone whether something seems convenient to them generally speaking.