I've noticed you don't separate "Principle Questions" into their own category like my first LSAT course did. Why is that?

It's because I think the only useful categorization is one that helps you learn to see things by type. And there isn't a type of "Principle Question." There are four distinct ways that the test writers construct questions with the word "principle" in them, and those four ways correspond readily to four existing types of questions.

Maybe an analogy would be useful here: 

Imagine I wanted to create a taxonomy of world religions. I'd group them by the core beliefs that their adherents share, right? And we'd have a list that went like, Islam, Christanity, Buddhism, Jainism, and so forth.

Now imagine I happen to have four different friends, all named Kevin. Kevin S. is a Christian. Kevin M. follows Islam, Kevin P. practices Jain, and Kevin R. is a Buddhist.

The fact that I know all these Kevins wouldn't lead me to change my religious taxonomy to now say we have Islam, Christanity, Buddhism, Jainism, and also Kevin.

Because "Kevin" is not a type of religion! It's just a name shared by four different people, each one of whom can be usefully grouped within one of our existing categories.

Same thing here.