I think the problem with this discussion is in the usage of the word "descendant". While you, Vainglorious, use it in the sense of biological reproduction or evolution, in TES it often means that the creatures in question were directly created by their, presumably divine, ancestors. So when people say that Argonians might be descendants of the Hist, it doesn't necessarily mean they think that the Argonians are plants of some kind; it means that they think the Hist might have created the Argonians like the Aedra have created the Mer. While I agree that it makes little sense to say Argonians are plants, the assumption that they were made by the Hist isn't at all that far-fetched (while in truth remaining just an assumption).
Actually, having seen it worded that way, I can now begin to truly understand where people on the "other side" of this argument are coming from.
If the Hist are divine beings of some caliber (and this is a reasonable assumption, since they are worshipped by the Argonians [an homage mortals don't usually bestow upon other mortals]), then it is therefore a logical conclusion that the Hist are the creators of the Argonians, in the same way that the Aedra are the creators of the mer (as you already mentioned above). However, as you say, this still does require assumption, since it is never explicitly stated anywhere in the lore.
As well, it is still unclear in the lore whether the Hist are powerful enough to create a race of living beings, and related to that, it's also not clear whether the lore regards the Hist as actual "divine beings." It's still quite possible that, worshiped or no, the Hist are not divine and do not possess powers of creation. From everything I've read, this issue has yet to be fleshed out, which is a real shame since I'm now extremely interested to know the truth.