Before I did anything I'd call the fire department and ask them what would be the safest way to go about this. A friend of mine just lost her whole home to a fire. Any cost you put to making this safe now is nothing in comparison to losing your whole home. I'm sure that's why you are asking this question. No offense, but I wouldn't trust anyone on this forum, unless they have the right credentials, with the safety of my family and home.

Someone from our GC cut the plywood. We had a separate contract with the granite fabricator but there was a line item in our contract with the GC for support of the granite installation. That meant that he had our project manager and a plumber there for both the templating (so everyone could agree on sink placement or any other issues) and installation. They did something similar to what Kevin showed by cutting away an area of plywood where sink accessories go through the counter rather than drilling a hole: I think that is a better way to do it. The threaded shafts on faucets, hot water dispensers, etc often aren't long enough to go through more than 1.5 inches of counter (2 cm granite plus 3/4" plywood) before the nuts are put on. By cutting away an area instead of making a hole the same size as the granite hole, they only have to go through 2 cm of granite and one doesn't have to fuss with extenders.

How is the electrical supply wire run to the receptacle and how did you install the outlet? Any way to cut through the wall for access? If you have electrical wire running to the top of the opening now, I'm wondering why you can't run a cable connection & extension cord from another area, make the connections and stuff the extra in a small opening above the mount. Depending on the span, I'd worry about the 2x10 holding the weight if it isn't secured in more places than the ends.

Ditto to all of the above. Do you really want to incur the risk of breaking the stone yourself? Around faucet holes is a fragile area, even for a professional. Sometimes, despite the best installer's technique, the stone will crack. At that point, you need a professional to fix it or replace it. If you do it yourself, you'll be paying any of those charges. Plus, I would imagine it would be more likely to break if you're not experienced at this. I know it's not what you want to hear since you don't like this company, but I'd definitely suck it up and call.