Thanks Sweeby... I actually gave up and just asked an expert - the plumber himself. Once I gave up on getting stuff ordered that day it wasn't quite so daunting... Oh, and just for anyone who may need this info in the future - I've been advised to do a thermostatic valve, plus 2 wall valves, 1 for the overhead and 1 for the handheld. Apparently that will do the trick. Yet another thing learned!

I second the suggestion of using a command hook to sweep it to the side just a little bit. I use some brushed nickel (look) hooks in my shower to hang wet wash cloths and they've been there for several years with no problems. They don't mar the tile and I think if you got one (or two) that are the same finish as your hose, they wouldn't be too noticeable. If your tile is light, you might find a light color that would blend with the tile.

Get out of the big box stores as you can certainly find diverters to create what you want in a real plumbing supply store. I have no idea why the strength of the shower arm is at issue since the shower heads are attached separately from the diverters. I am also not understanding why you would have a diverter OUTSIDE the shower since the purpose of a diverter is to enable a person to change the source of the water INSIDE the shower. I am not seeing any advantage in terms of maintenance or installation to having one diverter which is inaccessible while showering. My controls are completely accessible from outside the shower because they are near the entrance of the shower so I enter the shower when the temperature and source of water is what I want and then I can adjust while showering. However I think you still are not grasping that you need to buy everything from the same brand since the trim (shower heads and what you see) needs to match the rough (e.g. the actual diverters and other mechanisms). Pick the "trim" you like and tell your plumber or GC what you have selected and what configuration you want and then have him order exactly what is necessary. In terms of pricing, generally Delta and Moen are the best quality for the money. Delta is guaranteed for life and parts for repairs if necessary are always going to be available and will fit.

You need to check your local Code requirements. It is illegal to have more than one water source operating at the same time in California. Therefore you can only purchase diverters which divert from one to the other. At least in California you would not be able to pass inspection with a hand held shower that could be turned on independently of the fixed shower heads. Many of the plumbing supply companies - just like auto companies - manufacture to be in compliance with California requirements because it's such a major market. I am not a pro but just finished a gut remodel. It is my understanding that the rough plumbing - e.g. the diverters for example - need to be the same brand as the fixtures you purchase. When I selected my fixtures, this information was given to my GC for his plumbing subcontractor so that the appropriate rough could be purchased - not just the brand obviously but exactly the configuration that I was selecting along with specs for everything.