Sorry, but THE industry bible and controlling organization would rip you a new one if you leave 12" of natural stone unsupported. You are personally liable if that kills someone's Chihuahua. If you don't subscribe to using Industry Standards, you ARE going to have customers that want to know why, not just pesky online designers. So, why don't you follow industry standards?

The NKBA recommends 15" for a counter height seat, and 12" is only for bar height. A table height seat needs 18". ALL of those overhangs need support engineered into the DESIGN. Plywood ain't support. It bends. Stone doesn't. It breaks.

Design ain't rocket surgery, but I swear there are days that I believe that I could personally create the Mars capsule, and probably should since there seems to be so much bad design , bad reading comprehension, and bad math, just everywhere.

If you are in the industry, buy a darn manual and memorize it. it is your protection from losing your shirt in a lawsuit if you follow those recommendations. Yeah, it's harder than not bringing up ugly truths that the Kitchen Designer failed in her job. Tough noogies. A Pro has a responsibility to do the job correctly, no matter who has been there mucking up the job. Last person to touch it owns all the problems.

That pictured island wasn't designed properly for an eating overhang if that stone is floating in mid air. A 120"x 66" island will need a LOT of design work to make happen. And a Jumbo slab of stone.