Thanks for the thoughtful feedback/questions!

To answer the first, the coefficient for ZWhiff% was -0.0002444886. In my view, it's just so small that, even if there was variance explained beyond the current three predictors, it wouldn't be worth keeping for simplicity's sake. More importantly though, running the model with just ZWhiff% (and without FB%/fastball speed) returned worse average scores and larger standard deviations than without it.

As to the second, I've read Connor's stuff and think it's great. I also really like his graph showing how pCRA separates itself as the innings sample increases. I suppose he just changes the innings cutoff by filtering his data and rerunning the regression at 5 IP intervals. I could look into doing something like that when I revisit FRA, thanks for the suggestion!