That's a nasty seam and the front edge profile doesn't match.

Agreed, however I’m at the point where I don’t want these people attempting a 4th try. What recourse do I have? Request to not pay the remaining amount on the project?

@Wendy K Not sure you should have to let them back in. Do believe it is court time. Thank lots of pictures. If you have any before, in between and more after get them together.

I'm lucky to put the table in a box together, but do believe I could have done a much better job.

I am a fabricator. Corner joint is botched, seam is poor. The varying gap between the underside of the stone and the cabinets is normal but typically it's filled with caulk after shimming. A quality install would have the installers taping off both sides of the gap before applying the caulk so the resulting caulk bead is in the gap and not smeared all over the stone and cabinets. Many shops don't do the taping because it is time consuming. However, most will at least caulk the gap even if they only use the wet finger approach (no taping) to finishing the caulk joint. My suggestion is to tell them after 4 tries they can have their stone and you want all your money refunded. then go find a competent fabricator.