Chad is a genius, also I don't think your descriptions of who contributed what to each song is accurate.
It's impossible to summarise in a post and also it changed over time - at the beginning it was 90% Chad, 10% Pharrell... and slowly over time that balance shifted to 10% Chad, 90% Pharrell (or even 100% P lately).
Simple description is that P has ideas and Chad tidies them up and makes them sound good. Also he's a master of "sound" itself - so if you hear any unorthodox effects or sounds then that's also Chad. If it doesn't loop then it's probably Chad. And if a preset somehow sounds thick and full then Chad has gone in and spent time making it sound better. For example that Pharrell "Liverpool bass" sounds always feels much thicker and more realistic on tracks where Chad is also credited.
However, when they truly come together as The Neptunes and work in the same room it's definitely equal contributions - P's ideas mixed with Chads execution and you're going to hear that on NERD and Justin before the year ends.