I find once I import from BIAB into my workstation (GarageBand, for now), then add vocals and my own guitar work, that I can’t add any more tracks from BIAB and sync the tempo of the new track with the existing tracks. For example, I would have like to have played around with my Bass choice, but anything I tried to add was way out of whack.
What happens when you try to import the tracks? Are they mis-aligned, or is the duration off?
I believe that the .wav files that are output are ACIDized, so perhaps there's something happening there. Did you set the project's master tempo in GarageBand before importing the tracks in from BiaB? That might cause GarageBand to stretch them to a different duration - although why that that didn't happen to your initial set of tracks is a bit of a mystery. If you describe the issue (perhaps on the
Recording, Mixing, Performance and Production forum) in a bit more detail, I'm sure someone will recognize the problem.
I’m sure there must be a way to change the volume levels on different parts of a single track—e.g., where you want a back up guitar track to sound louder in some spots than others.
Yes, you can change volume and other parameters dynamically in GarageBand. Have a look at
track automation curves.
So any advice on the above, and generally on the song, that can help further my education will be, as always, incredibly appreciated.
There are dueling guitar solos at 2:20. I'd suggest keeping it down to one at a time, so there's nothing competing for attention. That, and stopping the song at 3:30 - you've basically got a one minute fade on the song!

Hope you like the tune.
Yep, I enjoyed it all right. Thanks for posting!