Good move re-checking air fares. Even if the airlines say no, it's always smart to keep checking and see what you can get. I honestly can't think of any time when I actually paid a re-booking fee.

I'm a bit confused by your directions to Anse Marcel. It starts off fine by going over the bridge and through Marigot, but I lost you after that. You would only use Rue de Pic Paradis if you were planning to visit Loterie Farm. Just keep going north towards Grand Case and then east towards the area called Hope Estate. At the traffic circle you take the road Rue de Cul de Sac and after you pass the pharmacy, etc. you take a sharp left onto Rue de Anse Marcel. Of course, thats where the fun begins as you drive up the mountains and negotiate the curves (but the views are really nice along the way). This will eventually wind up at Anse Marcel and there used to be a parking lot to the left where you could park, but I'm not sure what may have changed up there.

There used to be some boat chartering businesses up in this area, but I would suggest checking into that before you go. Not certain who came back and who didn't after Hurricane Irma. This is a very pretty area that may people who sit St. Martin don't take time to explore, but it's worth the time and effort IMO. Good luck.