^ Basically, and I really would trust him with something like that (who else would you trust?)
I have normally been the one doing all the testing, though.  

This account was out for 6 hours today, because I totally lost my marbles... in fact it was just now reinstated, it was still out before I began eating. Never heard anything from Shadow or anyone, but I knew it was back up because it stopped saying "Ex Member" under my name. I still think I know who to thank, cause I doubt my account restored itself 

One discovery I did make today, is that if you give a new account the same Displayed Name as one you deleted, the name displayed on the old one's posts (that it made under that name) will change to that of its registered username instead. Usually the displayed name for a post will change to whatever name the account was using at the time-- whenever you post, YaBB remembers what displayed name you currently have, even if you'll change it later, though if you make a new account with that name, the displayed name for the deleted account's posts will change to prevent duplication.

I also now know why you can't delete an account if the Displayed Name field is blank. This is because, if you walk away for your computer during a profile session, for whatever reason, it will "time out" after 5 minutes. If somebody else gets on your computer, like, say, your mischievious little brother, and tries to delete or vandalize your profile, and the session has timed out, YaBB will just submit a completely empty "form", which precludes any changes to your profile, up to and including deletion of it. That doesn't explain why, before you try to delete an account, it has to check for password mismatch, an overly long displayed name or one with banned characters, or an improper birth date. (Speaking of which, you can actually change your birthday to "February 31"...)

Characters you can't have in your displayed name include
{ } [ ] / \ ( ) < > ' " ` ~ $ & * !

This may be to prevent regular users (not mods or admins) from using HTML or YaBB tags to color their names. In fact this is actually how I discovered this.
You still seem to be able to use accented or non-Roman letters in both displayed names as well passwords. (The latter will cause problems if you are trying to log in from a 3DS, since you can't access the non-ASCII characters from its password entry)

Attempting to view my most recent posts just produced the message, "This user has 0 posts", although viewing my profile shows that I have over 3,200 posts.

EDIT: Making this post causes it to list the posts I made on this board. If I posted on another board, would it start showing the most recent posts this account has made there?

Perhaps this is little problem will fix itself in time as I post at least once on each board, which apparently causes YaBB to remember all my other posts on that board again. I'm not going to spam just for that, of course.