Glass and tile drill bit and screw thru the tile using a mini clip. Why would have a plumber install a shower enclosure call a glass company (preferably the company that measured and had it manufactured?

Sorry I think you misunderstood, its a shower panel not a shower enclosure. A shower panel is a panel that has a shower head and body sprays on one panel. To install it they will have to drill through subway tile glass to screw in the mounting brackets and my tile guy doesn't know how they will drill through the glass subway tile he installed without cracking it. That is why I was wondering if anyone had experience with this without cracking the tile and the best method to be used.

sorry but I still believe the tile would b drilled the same way with a diamond tip drill bit.

You've got the install sequence backwards here. You install the rough plumbing first, then do the tile, and then the trim fits on top of the tile that is cut during the installation phase to fit around the rough plumbing. You don't do all the pretty bits and then drill through them to do the rough plumbing, and I'm not really sure that you're going to be able to do what you want here without breaking a lot of tile.

hopefully all roughins were done prior to the tile/glass work.
if it has been they will use the existing holes (that the tile guy left for them to connect the supplys to. all other Hangers/grabbars/etc. will have to be drilled into the mortar joints.

The rough in is done and the tile is in as well. It's just a matter of connecting the brackets and to do so you need to drill through the glass tole to make a hole to screw the bracket in.