Hey Bonk,
Finally got the game working.... I'm not sure if this has anything to do with the error I was getting, but it seems the game doesn't like me editing the Metaassets files. Go figure....
Interesting. Glad to hear you got it working.

Which MetaAssets files were you editing? You should be able to if you do so correctly, the game is very fussy about the format. Use notepad and do not change the spacing. Also, if you edit \MetaAssets\ServerProfiles\SinglePlayer\MetaMap.gf then make the same changes to MetaMap.war.
Looks like the mss.dll was a total red-herring, lol.
What changes to the singleplayer game were you trying to effect?