The notion that men are all in on a devious plot to oppress women, rather than women's oppression simply having been the historical product of material forces (including biological differences)

Yes it is an attempt to explain female oppression in a way that doesn't put it down to biological differences and "nature". That doesn't make it a "conspiracy theory" It is simply a power structure within society just as racism is. I've never heard anyone say that there are a group of men conspiring to maintain patriarchy knowingly. Most of the men who benefit from it just see it as natural and the way it should be the same as any ruling class.

So them saying something like, "some women have penises, so your guide on women's health is offensive because it only mentions vaginas", is categorically the same as declaring that the word history is offensive because it (allegedly) implies history is about men and not women. The allegation here is that you started it.

No. Suggesting that any male can be a woman and thus "some women have penises" is offensive to women. As I remember it the coining of the word "herstory" was mainly an attempt to make a rhetorical point about the male focus of historical research at that time (which still exists to this day, unfortunately). It was not women literally being offended because the word history has "his" in it. It's just ironic given how male focused most history is.