...Should I change the final drive gear oil? If so, do I just fill it til it's full or do I need to meter out the recommended amount?.... I'm packing rain gear and plenty of thermal underlayers...I suspect it will be chilly in the Rockies in mid September...... Have a minimal tool kit with tire plugs.
IMO:
Final Drive: Sure give it a fresh change. It's easy and do the whole thing. Refresh all the fluids on a used bike, sooner than later.
Cold Wx: Thermal layers will only do so much, then you are
one with the cold temps --especially after more than a couple hours or so below 40-degrees. If you have the time to install an outlet and get a heated liner (I personally like the Warm-n-Safe with remote controller) you will thank the gear 10X over if you need it. Check CycleGear for quick help, expect $250 or so for all you need. Plus you save so much weight and bulk with one heated layer.
Cold Wx Plan B: change route if snow forecast. Go further south--a couple hundred miles can make a big difference--and some great routes there in New Mex. Maybe you can catch part of the SW-FOG FJR rally being held in the Taos area Sept 16-19. Clicky here:
SW-FOG info It is a blast to see and ride with other Feejers, even if just a day enroute.
Tire Repair: My minimal "sticky string" repair kit served me well far from home. Held perfectly till end of tire. Your compact bike pump will be fine, as long as it is reliable, as you mentioned.
Other: If using a stock seat and going >500 miles a day, wear Underarmour briefs instead of cotton and apply Gold Bond liberally, before during and after.
Have mucho fun.