I would say don't take French on to A2 - as has already been said, it will be harder at A2 and you'll need a lot of work to get a higher grade. I'm sorry you didn't get what you wanted, but if you got better in your mock, that's a good case for a resit or two (be wary of doing too many: you don't want to jepodise your other subjects) with a private tutor. An E in an unrelated subject really doesn't matter that much at all, but if you can improve on it, then great.
Carry on your best three - that's always the most common thing to do, and for good reasons. If you want to, re-take C1 and/or C2 next summer, as C3 and C4 build so much on the AS pure stuff that they'll seem so easy and you'll pick up the extra marks to give you a bit of a cushion for an A next year.
Well done for your A in art by the way - from my friend doing art A-level, I know there's a fantastic amount of work you need to put in, as well as having to be very talented in the first place! I'm very impressed (and jealous!).