The Pimple I should have the LED lights in by days end tomorrow. I love this shower! We backed down the flow from our new supply lines source. We installed two 1" Globe Valves and I found it took 11 wrist rotations to open fully or close it off tight. I calculated we needed to ristrict the water by 60 percent to make this 'Sculpture" safe. I gave the valves 4 wrist turns from a closed position and fired up the system. No change the shower was still flat out. Next 2. Next 1. Next 1/2 Next 1/4 turn. Amazing. I only opened these valves a 1/4 turn and the shower works like your every day unit. With the "Big Rain" the water is softer and doesn't belt you. Hope you like it.

also, if you look closely, this photo of a helix silestone counter looks like it has the glued edge. if that's the case, it might be less noticeable due to the grain of the pattern running horizontally. but it might be more noticeable in real life. the only thing I can tell you is to try to see if you can find a full size counter like that in real life. maybe go to other cabinet/counter stores and see what they have on display? or ask other installers to bring you samples of their work. get a second and third opinion. don't just go with what the first guy shows you.

These people have it https://www.wurthlac.com/storefront/decorative-hardware/handles-and-pulls/wooden-pulls/96-oak-pull/prodMRCPJ34K96.html

You are correct that those are edge choices, not stone thicknesses. If you purchased a 6cm built up edge, he needs to remove the island top and redo it. Your contractor will need to build up the island subtop by that 3cm and take into account the consequences of that extra counter thickness, like the additional counter height that it adds to the design. Overhang supports are created in the design phase and installed by your cabinet installer. That is not the stone installer responsibility. Support must happen before the template happens. 3cm needs support beyond 10”, so yes, you'll now have to retrofit something external like L brackets with braces, and likely reinforce the cabinet interior to accept it. Or, if the island is getting redone, that’s the time to have the designer reengineer the support correctly. Which they should have already done.