In writing with contractions, would it be correct to write: "Kounye a w' konnen." Or is it more native-sounding to leave it as "Kounye a ou konnen" without the abbreviation? Mesi ANPIL!

You may write it as: Kounye a  w' konnen. It's correct.
Others will also write it this way too:
Kounyeya w konnen
Kounya w konnen

Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words