So in the island like on the picture, tgere has to be cabinet panel first than Dekton? has to be cabinet panel first than Dekton.

Actually the same on the right side since both sides are exposed. There is a stone in that place on the picture, but looks like the miter it all the way around.

The material we are using is 2 cm

The material color I picked unfortunately not available in 30mm in US. There is a physical space to do mitered returns but like my original question was if I don’t do mitered returns the inside part of the waterfall panels will be unfinished. I hear different opinions from installers, some say it’s possible to smooth it to bring it to the color of the stone top (not the texture but color like edges are solid colors) but some are saying it’s not possible to do which means that part will look unfinished without full overlay return. So still undecided what to do.

Buy Roberto Colonatti's fabrication and installation manual and do exactly as he says please. He's the best in the world.

By internal steel support, you mean this... right?