For #1, I do think online learning could be a wonderful alternative. My DD is doing it this year in 5th grade and is loving it. We are also talking about next year's arrangement. She could take online geometry (offered by our district free of charge, but it's a "regular" geometry course), or more in-depth (more proofs) geometry with 8th graders. But we are also wondering about AoPS, because I don't feel that the "in-depth" geometry is really much better than the regular course, and most likely DD will be bored pretty soon anyways.

For #2, ask your district. If they have had dealt with similar situations before, they would know what to expect. We asked and they gave us a few routes that DD could go in middle and high school, including busing to high school while in middle school, and taking classes at local colleges while in high school. But for us it's a bit different in that DD doesn't really care that much about math. She takes advanced math these days just because it makes school days more fun than sitting in the regular math class (she does very little math at home and finishes most stuff at school). She wants to be a musician. So in high school, if she doesn't change her mind, our priority would be to ensure she has time to practice her instruments...