The example my book gives for an ungrammatical usage is "山田さんがくれば、いっしょに大阪に行きませんか？" I recognize that such things are the standard, but nowhere in any of my research is it indicated why that's the case, nor does it explain why sentences like "高ければ、買わない方がいいですよ。" are considered grammatical. Can anyone clarify this(as well as what falls under an ungrammatical will, wish, order or request, in this context)?
I think it has simply to do with the kind of nuance that is conveyed when a particular "if"-construction is used.
と suggests that whatever follows needs to be a natural, unavoidable consequence and a wish/will/order/request is not a natural consequence.
If Mrs. Yamada can come, then her husband won't be able to come. (Because one of the two has to watch the kids.)
I always think of ～れば and ～たら as a pair with ～れば being the non-past tense version and ～たら being a past tense version.
If it turns out that Mrs. Yamada can come, should we go to Osaka? (...to avoid having to meet her.)
～れば suggests that knowing whether the "if" part will be true is enough to follow through on the "then" part.
～たら suggest that once the "if" part has already occurred, one should do the "then" part.
If/when Mrs. Yamada has been able to make it here, why don't we all go together to Osaka?
So, strictly speaking, I don't think the wish/will/order/request with ～れば is ungrammatical, but the nuance is not the one you expect when you translate it as a simple if-then construction into English.
なら is independent of whether the "if" part has already occurred or hasn't yet occurred, so
If Mrs. Yamada can come, should we go together to Osaka?
is probably open for either interpretation (i.e. go all together or go to avoid her), but going all together is probably the more likely interpretation.