It's quite easy to get a majority Christian Lebanon without resorting to Egyptian genocides of the Copts. As has been said, have the French go with a more realistic Lebanon border. Have the French trade the area south of the Litani to Palestine for all of the Golan Heights and maybe some other concessions in Iraq. That area alone being gone gives Lebanon a much more sustainable Christian majority and makes the civil wars which really caused Christian emigration that much less likely.
Syria is quite a bit harder. I can't see a genocide of the Copts helping here, either, as I don't see why the Copts would want to go to Syria. The only thing I can think of is a near-ASB "Great man" leading the Syrians to conversion somehow....