The backsplash will more than cover 3/8" . Most thin tiles are that by themselves. Thicker tiles can be 1/2" or more. Then the thinset is anothe 3/8" at least. Don't sweat it and leave it alone.

This discrepancy should have been caught when the countertop guys templated the countertop. Are your walls that messed up, or is it because of a flawed cabinet installation? Regardless, a 1/2" gap is ridiculous. You can have a new top properly fitted. Bingo, problem solved. Is there a backsplash to be installed? That can hide a gap, with the exception of the gap at the end of the countertop. Depends on if that edge will be visible or not. Everything else is a compromise of sorts in that you can reduce the gap in the back but you overhang in front. You can slide it back but then your cabinet overhang at the front of the cabinets will be skewed. You can scribe the countertop as is, slide the entire thing back 1/2" to close the gap on the right. That'll give you even reveal on the front overhang, but it'll be 1/2" less front cabinet overhang than you have now. Or you can do a fraction of that 1/2". Slide it back 1/4" for a 1/4" gap on the back left, and lose 1/4" of overhang at the front of the cabinets. You can notch the drywall on the back right wall and slide the top back. Same effect as the previous recommendation, but you're notching the drywall instead of scribing the countertop. Another alternative is to use mud to float out the back wall. Essentially you're making the back wall "thicker" to bring the wall forward to meet the countertop. It'd have to be done in several steps, and you'd have to feather the half-inch thickness down over the run of the wall. But that also depends on windows on the wall and how the existing window trim will be affected. There are fixes and there are band-aid repairs. Mongo

Exactly what I thought, that the cutout wasn't large enough. We had some issues with two fabricators here in NJ when I asked about a positive reveal. They claimed they never heard of such a thing... Always installed sinks with an overhang. My installer in Iowa was fine with doing it this way a few years ago. I suspect that the difference is that cutting out for a positive reveal requires a more accurate cut. It's harder, so they "can't do it". Stick to your guns. Have then adjust the openings, or re-do. If the positive reveal was on the order, that's what you should be getting. Good luck!

I am buying new construction and they have had an issue where they had to remove and rehang the cabinets and countertops. after they finally reinstalled everything, I see a gap between the walls of one of the slabs of quartz. The other slab is tightly flush against the wall. what bothers me is this is the part nearest to the sink. is this acceptable?