I gave up on achievements in the end.
However, I do know that the dev for Repella Fella was able to get it working. One thing I noticed in their export was that they had not packaged assets. I'm actually wondering if that's part of the reason the wrapper wasn't working for me.
I didn't bother to see if that actually made any difference though as by that stage I was already over it, but it might be something to try?
***edit*** if you're referring to the game crashing on startup, I didn't have that issue. I used nwjs 0.71, Steamworks sdk150 and Armaldio's Greenworks files. All the GOG overlay stuff seems to work except for the achievements.
I was stuck on an old C3 version though (pre-Steam exports options) - r317-2 and had to add those arguments manually.