I have glass tiles but I guess mine are different. But I will say this if you get 10 glass door companies out there you will get 10 opinions on what needs to happen. The people that don't bring levels and such get rid of them first. They wanted to put support rods across the top of mine and drill into my floor. Here is how mine is constructed maybe you can tweak it to fit your needs. I have a 28" wide x 80" tall 3/8" thick door that has 45 deg hinges that swings off of a 1/2" thick piece of glass that goes from floor to ceiling. There are 2 clips on the wall and one at the top where this glass goes to the ceiling. This holds the panel steady. It is just siliconed at the wall and bottom, no drilling into floor. The other fixed panel is 28" wide x 80" tall 3/8" thick and has only one small attachment at the top corner the rest is siliconed in. It works beautifully and you see the true beauty of the glass not a bunch of hardware and head rails.
My Glass tiles are bullet proof as it is glass laminated onto porcelain and it is really tough to drill through but I don't think they should have a problem using the correct drill bit and possibly keeping it wet while drilling with acetone or water to keep it cool. This company might have had problems before and they are now nervous with the situation. I suggest that you keep getting quotes. It's a pain and slows the process but in the end positive results.

Thank you so much for your reply.

Our issue if you will, is that we live in the Caribbean on the tiny island of St John and there are no companies here that sell glass shower doors so we are ordering from a Stateside company and and having it sent down via container and hubby will install himself (no worries he is a fantastic remodeler and so far has gutted and rebuilt the entire home including jacking it up and removing all support posts)

I definitely pass onto him what you did - thanks soooo much

Piaa

diamond hole saws to drill for the mounting screws!

they are sized down to about 3mm and will be fine for drilling the glass. Spray a little water on them while drilling if it doesn' t have it's own waterbox. Some are sold as a kit with a clever, clear plastic reservoir.