"Question: will support post on each end of the 10ft overhang be enough or do i need another post in center??" No and no. You can't do it like that. You could have your fabricator cut slots into the bottom of the stone and fill the slots with stainless steel or composite rods set in a bed of epoxy or acrylic adhesive designed for rodding. 30" long at a minimum and spaced every 16" or so. Alternatively, you could run a 2" steel square tube between the three posts and set the stone on that. Painted flat black and set in 3" or so, it will be somewha inconspicuous. Check your door swings first, please.

ASSUMING a nine foot ceiling ? These are not for you unless you do three small!....... Why? With the medium @ 16 x 30. With both BOXES set in 24 inches from island ends, you will have 40 inches between the glass on the two when hanging, and 24 inches from glass to counter end on both ENDS of counter top. Great! But due to fixture height of 30", you will see NO chain if as you need a 36" clearance bottom of fixture to counter top, and the canopy on the fixture is 6" ! There's your 108! You'd be far better off with THREE of the small. Or a ten foot ceiling. Or..........a different selection altogether. You don't need full scale models......it's math. If it's a remodel with an eight foot ceiling the answer is NO to the fixture. Period.

For your kitchen island, if you're getting 2 pendants, we would recommend close to what you're thinking and doing them 30"-32" apart and at least 32" off from the surface. But for the size of your kitchen island, we would recommend 3 pendants and have them approximately 20"-24" apart. Please see attached images to get an idea of how far apart they should be. Hope this helps a bit!

Go with 3, unless the 24" pendants cast a very bright/wide light. Spaced correctly, I think 3 would look more balanced on an island spanning 10', with little liklihood of casting shadows. Try suspending cut up boxes over your island to better visualize 2 vs. 3 lights.