Electrical – Dishwasher ground

I only have 2005 NEC handy, but there's nothing there against using same circuit for both dishwasher and disposal.

210.23(A) Branch circuits, rated 20 or 15 amperes may feed lighting units, utilization equipment or combination of both. The rating of of any one cord and plug connected utilization equipment shall not exceed 80% of the branch circuit ampere rating.

Your local code could be different, of course. Also note, that your garbage disposal needs a disconnect switch

The instructions for the fixture are only correct for a metal box. If a metal box was used, the box itself would (should) be grounded. The bracket that holds the light would then be connected to the box, which would make the bracket grounded. Finally the ground wire from the fixture would attach to the bracket, grounding the fixture.

In the case of a plastic box, the box is not grounded. All ground connections must be made by connecting the ground wires together. So in your situation you are correct, you'll connect the ground from the supply to the ground from the fixture using a twist-on connector (or other approved connection).