I wish I lived near Memphis, I'd have 4 words for you.
I'm on my way!
Unfortunately it's a 5 hour drive or so, and I just started a new musical project myself.
I htink you should give it some serious consideration then do what your heart tells you to do. I think the best thing would be to concentrate on originals, you've been thinking along those lines for a long time and this may be the time to jump in with both feet.
The band sounds like it's about to fall apart, which I hate to see, but it looks like everyone else is losing interest. As someone already said, it may be just getting boring.
Back problems? Not everybody will do it, but I've played on a stool a few times because I couldn't stand up, and had to ask someone to hand me my guitar because I couldn't pick it up myself. It really sucks...but not many people will do that, only a true, dedicated musician. I think your bass player is one of those, since he has been playing although it causes him a lot of pain, but it may be time for him to give it up. There comes a time when you have to decide you've had enough...
Good luck, whatever you decide, and if you get a chance to come down this direction let me know, we'll be playing gigs before long and I think it would be great to have you there listening. Not sure if I can let you sit in, due to the 2 piece format, we have to really have our act together to make it work.
But don't show up without a guitar, just in case.

I'm a member of the BOMB SQUAD.
If you see me running, better catch up!
http://billy-griffis-jr.artistwebsites.com/