You're in the right ballpark that's for sure - and your experience with hard-to-decode error messages is not a new one either! Haskell is somewhat "experiemental" in this domain, as its only now starting to learn the features necessary to do what you want to do.
Having some compilers background I understand very well the difficulties in making sensible error messages,
especially when dealing with such advanced features. The point was more that I don’t know the basics on
how those features work (and what is involved in the job of the type checker) so I can’t even parse what
GHC try to tell me (skolem what?).
Thank you that looks very useful!