Holy cow, I wrote a book!
I had another undeletable Outlook folder, and
all my tricks for cajoling Outlook into letting me delete it failed.
I already deleted all the items in it,
emptied it of offline items,
deleted all the failures from my Sync Issues folder,
but still I couldn't delete the folder.
But I came up with a new trick.
I moved the folder out of its normal location and made it a direct
child of "Mailbox".
As a result, it was no longer part of the list of synchronzized folders.
I synchronized one more time, and then Outlook let me delete the folder.
As I noted before, I don't know if this'll work for you, but it's
another trick you can add to your bag.
(And I still have three folders that I can't delete, despite trying
both this trick and the previous one.)