@jkbrooks
Woah!
There's a lot in there!
Yes, I have done multi-day tours, but I use neither a Garmin (I use Wahoo) and RWGPS only rarely.
In essence, you want to be able to plot a route on the go and get it to your unit, right?
Then, I'd suggest you change your mentality a bit and not insist on using RWGPS all the time.
As you have discovered RWGPS has different functionality for Web and App (that also can change!).
From experience, using RWGPS on a tablet in Web mode is a PITA.
Strava is similar.
(But you can try all that at home).
It's a question of the right tool for the job and since apps don't weigh anything there's not much issue in bringing a few along.
There's also the question of internet connectivity. For most, no Internet no plotting.
To my mind the best solution for getting you out of a hole is Osmand. It will plot a route, work offline and allow you to save the gpx file for transfer to your unit. It will work every time.
However, the quality of its routes are not great, it's slow (offline) and it's a bit clunky to learn.
I don't plan routes on Osmand, but it has been my back up for years.
There's already lots of discussion about alternative planners. For me, cycle.travel was a godsend in the US.
I'm surprised RWGPS put you on a forbidden road (unless it was a temporary situation).
Have you tried looking on Youtube for a method to improve data transfer to the unit? It should not be as difficult as you have described.
Good luck!