1. Maybe none, since flow control is in the shower head, not the valve
2. It depends on the valve. Most, but not all, single handle valves can be reversed for back to back plumbing. You have to read the instructions to see if it can be done, and how to do it.
3. Not to a plumber.
4. It is a personal preference.
5. That is also a personal preference, based on what you like.
6. Few manufacturers make a built in diverter any longer. And few companies stock them even when they are available.