1. I think the diverter knob is what you'll have trouble using. (It's small and a little bit persnickety.)
2. Once you've used your shower a time or two, you'll have a good idea of the cold/hot mix - so at least turning the faucet on will be somewhat easier - and as someone else said, you use the switch at the shower head to pause the flow while you are showering (minimize filling your gray water tank), so you don't constantly have to fiddle with the faucet handles
Okay, one more thing.
I'm sorry your plan for the batroom faucet hasn't worked out; I get it. Once you get your trailer and use it a bit, maybe you can come up with a different faucet that will work more to your liking... And report back to us, of course.
Hang in there. I think you're still going to love love love your trailer!