This could damage the threads the adapters or the legs themselves.
Attach wood brace fto underside of granite counter.
The quartz counter was placed over the suspended sink and sealed glued somehow to the quartz.
Instead the counter should rest on an open framework of wood or plywood strips a couple of inches wide.
Screw the table legs into the adapters that are glued onto the granite.
I don t know if it was agreed on between the contractor and the quartz fabricator ahead of time but the quartz installers handled the entire process of placing the supports and the sink and then leveling the sink prior to the placement of the quartz.
Stainless steel and composite sink manufactures provide a system for attaching directly to the underside of the g q counter top.
I did this the simple way i bought some poster board taped it together to make it as large as i thought i wanted the table top to be.
The system includes 6 8 bolts clips and wing nuts.
Turn the table right side up.
One option is just to leave the granite floating on top of the apron.
The more exotic the granite the more fragile it is due to more quart fissures and voids in it.
Therefore at the mill a fiberglass mesh is applied to the backside of granite slab.
This helps to ensure a tight bond when joining the two materials together.
The bolts are inserted into grooves that the fabricator drills or cuts into the back of the counter spaced evenly around the flange of the sink.
I am a granite fabricator what you describe on the underside of your granite is normal an should not be cleaned or removed.
Make sure adequate supports are in place at the front back and sides of the cutout.
The weight damages wood and vinyl flooring.
What size granite countertop support brackets will you need.
Once you put the granite on the buffet gravity will keep it firmly in place.
Get the legs hand tight but take care not to over tighten the legs.
From the cutout edges but not closer than 1 in.
With respect to the cooktop cutout support strips should be in place not further than 3 in.
The size of your granite support legs and brackets will be based on your overhang.
You can attach wooden blocks to the underside of the granite with epoxy or construction adhesive to lock it in place and keep if from shifting side to side.