Having a glass ceiling as opposed to the fully enclosed shower shown (an insulated ceiling) is going to pose some challenges: the surface of the glass is going to act as a condenser and rain cold water. Maybe not a lot, and not noticeable with the shower running but if you stay inside to dry off you are going to get dripped with cool water. The other aspect is going to be maintenance of both the interior surface (which will dry with lots of waterspots unless you squeegee cold water down on yourself) and the outer surface which is going to be a dust collector that you are have to get on a ladder and reach over to clean. Its an interesting idea, but for me personally, the potential negatives would outweigh the potential pluses.

Is this screwed into sheetrock or into tile? Whatever it is screwed into will need to be repaired and the holes do not sound like they are moving very far so be certain that you do not break one into the other because then you will have a mess especially in sheetrock.

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.

That's exactly what I am getting for my bathtub and it's supposedly going to be installed on Friday. Whether it stays with no rail though depends on whether it can be made watertight. If it can't be made watertight, they will install a very low metal rail. My DH has the same door in his shower and he needed the little rail to make the shower watertight. Here is a picture of it. Here is something to consider. Your walls might not be perfectly straight so the shower door company might have to make a template to make the doors fit properly. Some off the shelf shower doors have a little bit of play in case your walls aren't plumb, but many do not and that is something to think about.