First thing, the glass company is wrong if they are saying that you should use annealed glass versus tempered because annealed is stronger. Annealed glass is not stronger than tempered, in fact glass is tempered specifically to make it stronger.

Second, annealed glass will not resist chipping easier than tempered glass will, but if annealed glass does chip then you end up with a piece of glass with a chip in it. Whereas if tempered glass chips you end up with an amazing number of little tiny pieces of glass all over the place. And while they aren't particularly sharp, your shelf will be gone and you will have a mess to clean up.

Whether to use 3/4" or 1/2" is as much an aesthetic question as it is a strength question. Will 3/4" look " too heavy" in that application? Will 1/2" look "too light"?

From the safety aspect, the 3/4" will be stronger than 1/2", but really as you want to use it, using resonable precautions when around the shelf should make the difference negligble.

You might try using a neutral cure, clear adhesive silicone to attach the glass to the stainless steel.
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