I did something similar using the Hansgrohe 7" rainhead in my tub shower for my hall bath. I have a Greek tub, so it is only 4 feet long. I use it as a tub/shower and I did not want any glass. We put the rain head in the ceiling in the middle of the space, so it is 24" inches from the end of the tub. The water comes out a little bit on to the step in front of the tub, but not much. So if you did the 15" arm, you should be totally fine. You are right about the rainhead making much less of a mess. If you take a look at my pics under "pics of my new bathrooms" you will see that we have no glass doors or shower curtains in either shower (just a small panel next to the vanity in the master). We were able to do that because of the rain heads.

Agree with what Mizlizzie wrote. I try to keep the bottom of the rainhead a minimum of 7' above the tiled floor. You can mix drop arms. Most are universal 1/2" thread. Generic finishes like Chrome are pretty much universal, you can mix and match from most any manufacturer. If you have a designer-type finish then sure, it makes sense to stay with within one manufacturer's product line.

I'd rather go vertical than across three walls and through two framed corners. If this is soldered copper and a remodel, soldering unions in a 3-stud framed corner can be a pain. It can be easier if the framing is open in new construction, or if the walls are lightweight construction, a partition end stud and drywall clips. If this is hard pipe, my overall goal would be to minimize the number of turns the plumbing has to make from the supply valve to the shower head. Secondary would be the shortest run of tubing. My own shower, I went across the ceiling. That gave me the fewest turns and shortest route.

Thanks much and beautiful showers! I am confused about the thermostatic valve. Is this something that I can buy separate and have it installed in the wall so that anytime I change out the shower handle/trim/valve it will automatically be there and still be accessible if it breaks and I need to change it? I hate to have more moving parts and things to break inside the walls. LIke this (although it looks like part of this would stick out of the wall?-another hole?: https://www.amazon.com/Rozin-Thermostatic-Mixing-Temperature-Control/dp/B01FAGWKTC/ref=sr_1_3?ie=UTF8&qid=1542006430&sr=8-3&keywords=thermostatic+control+valve#customerReviews  Or does do certain valves come with it like this?: https://www.amazon.com/Hansgrohe-04231000-Thermostatic-Control-Diverter/dp/B07FDFK5SM/ref=sr_1_1_sspa?ie=UTF8&qid=1542006430&sr=8-1-spons&keywords=thermostatic+control+valve&psc=1 I was wondering, also, it sets the temp but the initial issue of being cold while the water warms up since it has to run through the pipes would still be there, right? Problem is, I have chosen champagne bronze and if I need to get the thermostatic control valve built in I think that will be hard. I'm already having a hard time finding a handheld and shower head built into the same. I might have to get a separate handheld to divert water or I'm back to the initial question of do I just push the shower head over to shoot towards the wall before I turn on the H20 or go back to the idea of moving the control valve? Here's one of the options I was looking at for a showerhead before this discuss that I might need to adjust: https://www.amazon.com/58471-CZ-PK-H2Okinetic-4-Setting-Handshower-Showerhead/dp/B009MLXJN8/ref=sr_1_2?ie=UTF8&qid=1542006748&sr=8-2&keywords=shower+head+combo+with+handheld+champagne+bronze