I'll admit that I'm still very much in the learning stage, but my understanding is that the head term of a nominal phrase is always a noun.
I've been trying a syntax search to find how often each preposition governs each case, following the pattern given here: https://community.logos.com/forums/t/52262.aspx. I've gotten a lot of results, but never enough to account for all appearances of the prepositions.
I think I've tracked the problem down to the fact that a large number of nominal phrases have a verb as the head word (and verbs obviously don't have any case at all, so they don't come up in my syntax search).

I get over 3,000 results for this search, and I'm not sure what to make of it. Is this just an error in Cascadia, or is there something I'm missing?