1) Ran 3/3 gauge off the main panel to a 100amp SquareD3/3 + a ground wire?
'QO' type commercial subpanel.
2) From here he ran 12 dedicated lines to the home theater
room. Problem is he used 10/3 wire instead of 10/2. He
had the wire running through metal gang boxes. The
receptacles are yet to be installed.
He ran 12 individual runs of 10-3 w/grd?
Answer to question a)
If you are using plastic boxes there would not be any need for isolated ground, (IG), type recepts. Even if you had used a metal box isolated from the other metal boxes with only one dedicated circuit per box, there would not be any need for IG recepts. IG recepts are usually used when metallic conduit and metallic boxes are used and the customer wants the equipment ground of the recept isolated from the ground of the conduit because of noise.
As for going to the plastic box over the metal many recommend such.
As for the bare ground wire not being used, now, I would probably still terminate it on the ground bar in the sub panel. Floating it could act like an antenna. Bonded at the panel it will just work like a drain wire in the NM-B 10-3 w/grd cable.
b) your recepts are fine, no reason for IG recepts.
I don't think the electrician installed a separate ground rod or tied into a copper pipe....NEC requires the equipment grounding feeder conductor shall be run in the same raceway, or cable, as the feeder conductors that will feed the sub panel. The equipment grounding conductor must terminate in the same panel the feeder conductors are fed from.
All supplemental grounding electrodes, ground rods, must connect to the main grounding electrode system of your home.
c) What are the benefits of 10/3 over 10/2 if any? Is it just that it can be tied back to a panel with its own isolated ground bar....and what advantage does this isolated ground bar hold if, when it is mounted on a sub panel that is connected to a main panel? This question I pose in the absence of a separate grounding rod.
In your instance none. You just paid more money for the 10-3, unless he sold it to you for the same price as 10-2.
You use the term isolated ground bar.... Please be more specific. Is the ground bar isolated from the sub panel enclosure by insulated stand-offs?