Figured it out!
I wiped things out and did a fresh install then ran the importer again, but from the command line, and the errors were slightly different (Symfony in the later steps instead of Mvc), but I also had a chance to go through the .php files this morning, which I didn't have time to...
RuntimeException: Unable to copy file
/var/www/offsetguitars.com/public_html/forums/files/ to internal-
data://attachments/0/1-d82d1345d9807d06b96ba66ae4b2c303.data in
src/XF/Util/File.php at line 226
1. XF\Util\File::copyFileToAbstractedPath() in...