the only times we did use support for sinks was for cast iron sinks, which were really heavy, like 90lbs. We never came cross problems like this, we didn't have anything else to use. So we used 2-3 2x4"s. And built kind of like a frame to support the sink in case it gives the granite too much pressure. For your case, since you are using stainless steel sinks, it's not super heavy like cast iron, it will hang by the plywood. Add cleats on the side of the cabinets so that cabinet actually handles part of the weight. But now the market has something called undermount sink setter, I never used my self, but it looks like a easy tool, search around and see what other people say about this, and maybe you could use this.

Do not get a top mount. Ever. In my opinion they are awful-looking and impossible to keep clean around the rim. On the other hand, a positive reveal has some benefits, including less dish and glass breakage (the sink is more forgiving than the granite) and less likelihood of chipping the granite around the edge. My sink edge is flush with the granite edge, and after 12 years, I have lots of dings along the edge of the granite from where DH carelessly bangs the cast iron pots against the edge. I am getting a positive reveal on my next remodel.

Scratch resistant isn't acrylic. They mostly look like crap within 6 months of use. NOT suitable for undermounting. Only "suitable" for when someone is "updating" a kitchen to sell and are on their way out the door & will never really use it.

Grab that stainless flap with vice grips and wriggle it twenty times. It will break off and you can mount your faucet. Or tap it flush with a hammer and make a continuous non-compressive waterproof shim (v32 plastic laminate), hot melt it in place and install the faucet. If your sink has no mechanical fasteners, I like your plumber's idea. After removing the steel, have him reinstall the sink with a Hercules Universal Sink Harness. If it's his first one, it will take about 2 hours.