There are many devices that work but are not on the officially supported list; I suspect the Glo is one of those.
There are some devices on the supported list that do not work with ADE2.0 (but do with 1.7.2).
Yes, you can autorize multiple eReaders. I have heard tell of up to 6 devices (eReaders+computers) total, and of up to 3 computers and 3 eReaders. I do not know what the exact rules are.
Unfortunately, you can't check online to see what registrations Adobe thinks you have, or to remove dead ones. If you go over the limit, you have to contact Adobe (eg Live Chat) who will reset the lot, and you then need to reregister the ones you still need.
Chatted with Support and the answer is that the Kobo Glo is not supported and there was no fix for my issue