Usually the shower valve/diverter assembly and the handle/trim for that assembly need to be the same manufacturer, but the shower head and wand do not have to be the same brand. There are also configurations where the diverter is a separate item from the shower valve, but these are usually for more complicated layouts (wall jets, etc.). Bruce

I think you are asking about the expansion plug used in place of a threaded clean-out plug. Odors can drive one nuts when they have eyes and nose on site,I don't even attempt locating them from 1k miles away. Water heaters often impart a sulfur odor to water. This isn't the silver bullet you were expecting but here's a test. Double bag a 20-30 gallon trash can with over-size plastic bags. Sit can on stool.blocks or other support tall enough to allow tapeing bag opening over shower head. Run shower the minimum time it normally take's for odor to be detected. If no odor is detected,,,,,,,,until bag is opened you've found the source. If no odor before or after opening bag,empty can into drain to see if odor is then detected. Report back with results and we can talk about remedies.

2" is standard.

Personal opinion: a fine DIY solution. It likely will not give you any trouble for a long time, and when it does, it will be fairly easy to service. Not acceptable for a professional. This is like having a builder install a faulty outlet and throwing you a few extension cords to solve the problem. A plumber screwed up somewhere in the walls, maybe teed off a cold instead of a hot by mistake, and your builder doesn't want to fix it the right way because it will be a huge pain to trace the mistake back to the source. Not your problem.