1. Can I replace the boot from above, or do I have to remove the
disposal unit to do so?

below

2. How does it detach from the sink bowl? Is it part of the flange
between sink and plumbing?

no it doe not detach from the sink bowl, and is more of a part of the
disposer. IIRC it's more like a gasket between the disposer and the
mounting flange, held in place when the lower mounting ring is snugged
up.

3. Is this going to be a huge pain in my, um, side?

could be, depending on the plumbing involved...

Good luck to ya!

Darwin