Yes, but I wonder if, paradoxically, being so unclear actually turned out to be an advantage for D&D. Different folks saw different things in the game and thus it maybe was far more successful than it would have been otherwise.
I have the books. I've never run the system "per se", but I've lifted some stuff for AD&D. It's post-UA AD&D on steroids. If this had been available in the late 80's, I think that my brain would have exploded with joy... :D
Nowadays, I prefer things to be a bit simpler.
Yes, I can fully empathize. However, I must admit, that back when I bought it, I really didn't have a clear idea of what Castle Greyhawk was supposed to be. I was still very disappointed in the module and I thought that it was extremely lame. I've never used it.
I guess they were somewhat rare even back then? I don't think that I have ever seen one of them here in Italy. In those years, I also used to go fairly often to the US and I would go to gaming stores and I didn't find them (or, maybe, saw them and forgot about them).
It is actually fairly detailed with 2 1/2 pages of rules and some optional guidance on using hexes.
True. In general, I don't think that it is easy to have decent mounted combat rules in the framework of 5e rules. But I'd love to be proven wrong from some third party supplement!