I have 3cm granite installed over base cabinets with a 10.5 inch overhang. The total width of the granite is 37 inches with a run of 78 inches.
Question 1. Do I need support? I have seen several sites that say anything under 12 inches, support is not needed.
2. Since the granite is already down how can I hide them in the cabinet? I have three L Brackets that the store recomended but since they are being fastened to the thin cabinet wall what is the best solution?
You don't have a big overhang but IMO anything over 8" should have some support. You can either use the L brackets or some decorative corbels. The strength of the back doesn't really come into play since if your granite is going to break, its likely going to do so with less force than it would take to break the back of the cabinet. If you don't want to see the support, you could get some 1" square steel tube or some 3/8 x 1 1/12" steel plate and epoxy it to the underside of the cabinet. This would require some small notches in the back of the cabinet. I've seen the steel used after the fact but its better if the fabricator could have epoxied it INTO the granite.