Squashua84,
All good ideas. I went ahead and tried all of your steps from above – several times I might add. But to no avail...
However, I did find a kludgey workaround while I was experimenting that may help others.
First off though, one thing I noticed different was my mail folder was ~/Library/Mail/V3, whereas your folder was named "V2." Not sure if that means anything or not.
After trying all of your steps above, and failing, I accidentally came across a solution. First off, I deleted my Signatures folder to start fresh. Create my 4 signatures, plus 3 extra "dummy" signatures to play with. I normally only have 4 signatures in my mail, but I wanted to experiment with more.
By accident, I discovered that when I added all 7 signatures to all 6 of my mail accounts, BAM, the signatures all of the sudden showed up in the signature dropdown of my "problem" account. What the heck?
I also discovered that if I went back and started removing the extra test signatures from the other good accounts, that at some point (usually after the second account paring back to 2 signatures), I would discover that the "problem" email account now only shows 1 of the 7 signatures that are associated with it. Very very odd. Then after paring down the rest of the extra signatures from my good accounts, all signatures dissapeared from my "problem" account's dropdown. I know, this makes no sense at all.
So then I went back again and just added all 7 signatures, including my dummy test ones, back to all 6 of my accounts, and voila, there was my signature dropdown selections on my "problem" account. Not ideal, but at least now I have signature options.
Lastly, even though now I have signatures available to the "problem" account, there is still one bug with it. Even if I set a default signature for it, it will NEVER show the default signature when I start a new email, I still have to manually choose it from the signature dropdown. All 5 of my other accounts correctly show the default signature I set in preferences when I start a new email. Small consolation I guess is that at least I can pick the signature from the dropdown menus now for the problem account.
This is the strangest and most erratic behavior I have ever seen? It's almost like there is some weird low level corruption on that email account. But I cannot fathom how the above steps I took "kind of" solved part of my problem. Very very strange indeed.
Hopefully this will help someone else in my situation, solve their problem.
-- Jon