My group had problems with the location of the Arma2Net dll's in the past that caused what you've described.
The mso release had some debugging tips, did you have any luck with that? We didn't get any output whatsoever (potentially because I do not know what I am doing!), so I made a guess that the database interface code simply wasn't loading (as opposed to crashing, which would generate errors), and trying the dlls in a few places eventually worked.
We have copies of the Arma2NETMySQLPlugin dll and MySql Data dll in:
ServerDir\ at Arma2NET\AddIns\Arma2NetMySQLPlugin\
ServerDir\
We have version 6.5.7 of mysql-connector net, and 2.2 of Arma2Net (I think newer mysql-connectors should work, but it may be worth trying the older version if all else fails).
You mention running with a bare minimum. My experience is that MSO tends to need quite a lot, so be prepared for it to run badly unless you get good hardware.
Edit:
From the Arma2NET docs: "Arma2NETMySQL looks two places for the relevant files and folders. First, it checks the Arma2/Arma3 root directory. If it can’t find them there, it looks in the AppData folder. " -- this solves our problem, hopefully it helps with yours