You can block out between the studs if you are concerned. Some would call it overkill, but if you question it, block it out. Just be careful to get the blocking flush, no protrusions. One tape coat is code out here before veneering over any wall/ceiling with another material. No metal corner aid needed unless you lack confidence on getting a straight corner with the tile. In that case, the metal will give you a decent edge, but it needs to be installed correctly.

Usually that joint will get grouted, and if there IS any mastic or thinset peeking out, it needs to be cut back so it won't show once finished. You CAN, if you'd rather, caulk it, but it's not necessary.

1) 1/16- 1/8" sould suffice. Also, don't use silicone. You're better off with siliconized latex caulking made by the same company as the grout, in the same color and texture, so as to match the grout. 2) Use sanded grout. I doubt very much that it'll scratch the glass. If you're worried just be gentle when working the grout into the joints around the glass. But with joints that size, you don't have a choice. If you use unsanded grout, and you'll have problems with shrinkage and cracking.

I'm not sure how I came across this post, I think it was displayed on the right side and I clicked on it being intrigued by the tile making. I was at one time interested in working with papercrete and hypertufa. But after enlisting my brother, who wasn't as enthusiastic, and my niece, who poured cement down my laundry room sink, which original to the house had lasted 70 years, and ruining three blenders, my interest waned. True to form, if I come upon difficulty in a project, I think of it as a sign for me not to continue. And really, after all these years of trying various crafts that result in crap, I should know better. I also should have known better than to ask my brother, who always says it can't be done to any of my unique design ideas. And I should have known better than to ask my niece, who once stayed up all night before having to drive to LA for an art show, cutting mat after mat after measuring once, twice, sometimes ten times, but always coming out with the wrong size. I think we both have math dyslexia. I should video us trying anything crafty to see where we go wrong. I think both of us having ADHD might have something to do with it. Sorry for the backstory and run on sentences. So, I just wanted to see if you continued with your venture. And to say sorry for whatever tragedy occurred to cause you to have to move to Florida. Not that there is anything wrong with Florida, but it isn't Panama. Which is one of my favorite places in the western hemisphere. I hope no one is upset that I brought up a seven year old post.