Still can't figure out how to read messages in numbered order one afterI don't think there is.
Mark, please take that as a feature request.
I keep forgetting to mention it, but I often use Message View and having a Next and Previous that operate in message number order would be handy. Probably similar for Next and Previous topic in "Thread View".
In Classic Yahoo Groups Next/Prev were simple links, as were the message subjects in the Messages list; so browser-history link coloring worked well enough for users to distinguish read and unread messages. Mostly. I don't know if that would work here, and take some pressure off of the "Keep track of read/unread messages" item in your to-do list.