Answer

Festive greetings,
Many thanks for the compliments (I am not sure I deserve them).
As for your main point, indeed I assumed here the reason commonly given today. You can also see it in the explanations given nowadays (all those who report that even today grains of leaven are found mixed in with legumes. These discussions also came up in the talkbacks).
It is possible that originally what you describe was the basis for the custom (I have not yet read the sources you sent), but even if that is the basis I do not see it as a binding reason. And the fact is that reasons for this custom have multiplied, all of them after the fact, which suggests that none of them really holds water (I am reminded of Rabbi Meidan and his well-known saying that he knows 22 explanations for reading the Book of Ruth on Shavuot, but only one for reading the Book of Esther on Purim. Maybe I brought that there? I no longer remember). After all, according to this view one should divide legumes according to what produces hardened leaven and what does not, and not according to the question of what existed in their time and was included in the custom and what was not, or green legumes and those that are not, and all the other distinctions people make in this context.
As for my halakhic position, you are indeed right. On the face of it this is not the accepted position, at least not on the declared level. But if you examine things more deeply you will see that it actually is the accepted position. In the end, everything after the Talmud can be disputed, except that things have inertia and different weights. One does not change something that has become entrenched so easily, but there is no mandatory authority here like there is with the Talmud. Rather, in the standard discourse people enthusiastically declare that additions to the binding halakhic canon have accumulated over the generations, and do not notice that these are only additions of weight, in the accepted legal terminology. That too I accept in principle. This is not merely semantics. It matters, because when there is a significant need to change, there is no reason not to do so. After all, there are clear sources for authority-based arguments (“do not deviate,” “they accepted it upon themselves,” perhaps acceptance by the nation, and the like).
But on the level of principle I am not so impressed, even if that was indeed the accepted position. The more important question is what is correct, not what people do in practice. The desirable and proper, not the existing reality. Though of course what has been accepted does carry weight. Like the rule of “the generally accepted approach” in the topic of one who errs in judgment and the like (I should note that I discussed that too, and argued that contrary to the accepted approach, “the generally accepted approach” is only a sign and not a reason. If something is indeed correct but did not become widespread, it would have the same status, and one who errs about it would still be considered to have erred in judgment. And if something became widespread but is not correct, one who errs about it would not be considered mistaken. Like a foolish custom and the like). Beyond that, even accepted positions come into being in some way. If I manage to persuade the public, then my position will be the accepted one. Rabbi Blumentzweig once said that in our attitude toward custom there is an inherent contradiction, because a custom is created by deviating from what is accepted (the basic law), and from the moment it is created there is an obligation not to deviate from it. It is the freezing of deviation. Well said indeed.

Festive greetings, and again thanks for the comments and compliments,