Any luck? or ideas? 
For what it's worth, I firmly believe that Multibit's way of storing passwords is corrupt. Once I recovered my friend's password, he told me it was based on the password he had used but had some unreadable characters in it (the kind you can't type on your keyboard). Even when pasting the password from my software into Multibit's GUI, it kept saying it was the wrong password. I had to do it a whole different way through scripts, openssl, CUDA...
That I think is the same boat the author and myself am currently in. HD also had the bug as well in it's .5.1 version.
I would try another copy of the original in case somehow 0.5.1 tainted that copy. Try to open it with Multibit .5.0
0.5.1 had a known bug of UTF16 having characters dropped and can indeed result in a password getting mistranslated into other characters. It is known with the exact version you are using e.g.
https://pascal-bergeron.com/en/posts/multibit-corrupt-password/
Look at the section regarding "When things don't go so well"
Some of the most basic recovery attempts in this case was to try opening with the prior version. Hopefully it was just a .5.0 version wallet considering you stated you were able to open "both" wallets but couldn't send. If you got in initially, that may actually have been the case because if it were the .5.14 bug in the software you would not have even been able to get into that wallet. If .5.0 gets you in....export your data..words..etc and recover with the private keys in something like electrum.
Hey if trying to open it with a prior version works...Share some love
On this same note...Shouldn't someone have been able to make a script by now that would take a user's known password and simulate the bug in order to find out what it became?