I am a fabricator. A plywood underlayment for 2CM is typically complemented by a 4CM drop edge which masks the underlayment on the edge. the plywood is really just a spacer as it does not contribute significantly to the structural integrity of the overhang. For 3CM quartz a "fussy" fab shop will grind the underside of the overhang to remove the printed Logo's from the OEM. If requested the fabricator can polish the underside but the fab shop polish will not be of the same quality as the surface. However, it will be good enough to look finished on the underside. Polishing the underside is NOT an option typically offered by the fabricator. In my business we've learned to ask the question when the layout of the room, and particularly the stairway location, means the underside will be readily visible on a regular basis and not just when someone is on the floor looking up. Another option is to make the overhang "double thick" where 2 pieces of the engineered stone are mated back to back so a factory finish is on both sides. This is an easy option for 4CM assuming corbels are used or the double thickness is extended back over the cabinet. Finally, some natural stones that are crack prone have a fiberglass matting epoxied on to the back of the stone to help it hold together during transport, fabrication, and install. A competent fab shop will remove this backing as a matter of course. I am always annoyed at the poor quality evidenced by a job where the backing can be felt and seen on an island overhang and even on the narrow countertop overhangs. If you really want a great looking underside the best way is to double up the material back to back. It's an expensive way to do it for a variety of reasons.

Thanks. Yes, it will be cantilevered, but on four upright legs rather than two angled legs, although if I had known we would likely have to drill into the quartz, I would’ve gone with angled for cosmetic reasons. Four seemed sturdier. This is probably a dumb question, but should we be afraid to drill into the quartz- is it risky? Is this something best left to a countertop fabricator?

Thank you to everyone who responded, very much appreciate your iput. The owners of the company agreed to replace the counter at their cost and do it correctly - this was achieved only because we could back up our claims of an incorrect install based on the information provided by you all! Sammy - unfortunately, my sister lost her two dogs and cat in the fire; she was not home. The cause of the fire was the slight reverberations of the built-in microwave caused a screw in the mounting bracket to slowly over time rub the the casing off an electrical wire in the wall; the fire investigator had never seen this before and is now using this in their training. She would trade all of her possessions to have her pets back, a year later it's still devastating for her to have lost them. Again thanks to all and hope everyone has a happy, healthy, safe and prosperous New Year!

Not uncommon to box off that corner area. All the more common since cabinets went from being custom site built to prefabricated boxes. You could utilize the space if you have access room on the back side. I would remove the pony wall--gaining a bit more space, cover the back of the existing cabinets and custom build a face frame cabinet to fit in that empty space. That approach would maximize the available space. Alternatively you could slide a 24" wide prefab cabinet into the space and use fillers. Keep in mind that if the counter extends beyond the wall (like a breakfast bar), accessing the new cabinet will be a bit more of a head banging challenge. Since you are spending for quart$ counters, I would do the cabinet properly.