Can granite countertops rest on plastic corner braces

I wouldn't just try to attach the legs directly to the granite (or really any material used as a work surface). The result will probably be quite unstable.

I'd build a free standing frame and then fix the worktop to that.

There are some distinct differences between Silestone and natural granite. Granite is cut from slabs and always has some color and patterning differences if you look closely. Also, if you can see the bottom of the slab at some point under the counter top, you will see the machine marks (groves) made during the cutting process. Silestone on the other hand is man made from granite chips and resins. A close look will not show the longer natural strands or runs and irregular shapes of solid colors as in natural solid cut stone. Silestone tends to be a bit more homogeneous in its overall color mix, especially seen on larger pieces.

Practically speaking, natural granite is a bit more porous that Silestone, and needs to be sealed occasionally to keep contaminates out of the tiny pores. Silestone is manufactured to be extremely smooth and non-porous and does not require sealing.