You're doing an excellent work, about your questions, my two cents:
1- In my case I usually use 100 and 120 grit, just to let the plastic as smooth that It doesn't have splinters or pikes .
2-I use a vacuum just at the end, but make tha sanding over a plastic container to collect the maximun mess as possible
3- Rubber Blaster is definitely the "worry free." but more expensive, Doopy is cheaper but you need to be careful and I would rather don't let others to handle it.
Cheers