So I did the disk repair, and there were no reports and no permissions to repair... but I told it to repair anyway.
Logged back in no change in mail, still no response to anything but menus when starting at login on Full Screen. Changing the screen to window resolves the issue (for the time being).
So I turned it off Open at Login, logged off, logged back in, started up Mail, changed to full screen, everything works. It did do something a little strange here. The screen that it had been in turned that grey grid background with no menu bar... and remained that way while Mail was full screen. I should've made notes, but now I can't remember if I left that during the next restart, but it resolved itself as well.
Turned the Open at Login back on, made it full screen, logged off and logged back in and it launched full screen, and works normally now.
Some anomally within Mail, apparently, that has now been resolved, but I cannot say for sure what part of what I did fixed it... 😕
Whatever the case, it seems to have been resolved with my system. 😀
Thanks for the suggestion.
Hope this helps Zokora...