Ijma Of Usuliyyun
the knowledge of Ijam and Tawatur is the the present Ijma and Tawatur. absence of any disagreement confirms that there were no disagreement on it, or else, Ijma was not possible once there is difference.
Speculations have nothing to do with reality. We bother only when something is reported. No such report is there.
It’s not speculation . You are assuming all events that occurred were recorded and have reached us
These examples dont apply because these are reported (by Quran), and differences are continued to date. People still argue to date whether Christianity is a misrepresentation of true religion or not. The argument is there, and they are based on rationales. Even if the differences are cleared out, history will remember something like this happened, just because it is a matter of masses now, not just a separate branch of knowledge succession. But is there an argument in the terms there were 3 obligatory namaz versus the 5 obligatory namaz that we follow today?