You can use any pot to ground wires, as long as all the backs of the pots are connected together and grounded. The only thing you want to try and avoid doing is creating a ring with the ground wires. I'd definitely ground the bridge pickup on the bridge volume pot and the neck pickup on the neck volume pot.
I agree that the wiring diagram isn't optimised for an
ES-3 and the way it's wires run. It was probably one of the first diagrams done and whilst technically it works, it's not very ergonomic!
Definitely make up a cardboard template when wiring the pots together, and mark on the position of the F-hole so you can arrange the wires to run around the outside of the F-hole. You may find that taping or cable tying some wires together is necessary to keep everything neat. Also consider how the pickup wires will run to avoid being seen.
Draw it out, and post a photo of it if you want it checked or commented on.