We don't know how well Wickham and Mrs. Younge knew each other, only that they knew each other. Yes, we are told that Wickham went to Ramsgate specifically to meet Georgiana, so Mrs. Younge must have at least told him they were going there. How much they discussed beforehand we don't know. Yes, I think they would have claimed it was an "interesting coincidence" that Wickham happened to be in Ramsgate at the same time.

Your suggestion that Wickham may have planned to carry Georgiana off to France is interesting. I never thought of that before, but somehow, I think it would have been safer and more practical for them to stay in England (or at least in Scotland, if Wickham was planning on eloping to Gretna Green). We have no indication that Wickham spoke French or had any connections in France, he had not much money, and remember that England and France were at war during this time.

Yes, it was just good luck that Darcy happened to come for a visit a few days before the elopement was supposed to have taken place.