You're right about "I wish I
could buy ...". "could" is used this way all the time.
You're also right that wish+would can imply irritation, and probably usually does, but the degree of irritation can be strong or mild or anything in between, depending on the context. Consider that we usually are a bit irritated when we want someone to do something - when we think they should be doing it - and (Remember that these propositions are all counterfactual) they have certainly not been doing it.
"I wish the weather would be nice" is definitely not the thing to say! In the ears of a native speaker it is too close a parallel with "I wish you would be nice"! It seems to treat the weather as an animate being. It invites the listener to think in terms of "a nice person" rather than "nice weather", two rather different associations with the word "nice".
No, the writers of grammar books aren't crazy! They need to limit themselves to what is most practical for most people. You would need to get into linguistics if you wanted to learn the subtleties. I am only an amateur linguist myself, so I don't say that I'm always exactly right. Rather, I like to explore questions of syntax and usage in a certain way, and I talk in those terms as a sort of model of another way you could explore language on your own.