Hello, Ken. Have to confess I 'love your spirit'.
You got it right. I did this mini-march at least 4x.
So did the upgrades to the 2.6's go off without any errors? Got them to 3.5.1 with no issues?
There were some issues regarding PHP version and data format. So, at some point, I had to change PHP version and php admin/cli/mysql_collation.php --collation=utf8mb4_unicode_ci in order to be able to continue.
Did you check any of the courses for course formats addons after those 2.6's were mini-marched to 3.5.1's?
I might not be looking at the right place, but in the plugin overview there aren't any addons (plugins?) for course format. There are, though, for mod, blocks, enrol and theme.
I am hopeful that at some point a blessed guess will help us out.
--
Erackson