1) Since they usually remove the old plaster, I think it is ok to leave it. They will probably power wash anyway.

2) Not a big deal: http://www.poolinspections.com/manua...out-drains.pdf

3) You could probably seal the cracks but it depends on how large they are.

4) I would suggest changing to PVC but the size depends on the applications. In most cases, you shouldn't need more than 2.5" unless it is a high volume spa (> 6 jets).

5) Depends on the condition