C of G, I'd post the valve/rim Q on 'know how' and see what folk come back with.
Before I had the MTB I had an old racer and would have all sorts of probs with presta valves, mainly down to my own clumsiness, bad fitting and horrible cheap rims.
I have made temp repairs to splits where the valve meets the inner tube by cutting a small 'X' in the center of a self adhesive patch, enough to push the valve thru the center. You then slide the patch down over the valve to the tube and press it down making sure you're covering the split.
I stress this is only a temporary fix, and you mustn't tighten the valve
collar too close to the rim afterwards but with any luck it will hold air and get you home.

B)