The Tosefta on P'sachim [Pischa] states that after one leads the Seder, if he cannot do Hallel, then they met in public at shul and said Hallel as a K'hillah.
How can who is someone too ignorant to recite Hallel, successfully lead a Seder?
And this dilemma had bothered me for many years.
Then, via Hashgachah p'ratit [after all I am a mashgi'ach] I heard R David Halivni [RDWH] speak during a Shabbat Hagadol D'rashah as follows.
Why was the passage of Arami Oveid Avi [and its Midroshim] chosen for the core of the Seder? After all many other Torah passages relate the Exodus, some of them more robustly?
Answer: every farmer [peasant?] knew how to recite Arami Oveid Avi. After all they had to recite the vidduy when bringing bikkurim and so it was something they had to learn. Following Sh'ma, it was probably the best known passage among Amei Ha'aretz in Ancient Israel.
This approach answers my question, too
How can an Am Hakaretz lead a Seder and not Know Hallel?
Answer: every farmer [peasant?] new how to recite Arami Avi due to the Vidduy of Bikkurim. Following Sh'ma, it was the probably the best known passage among Amei Ha'aretz in Ancient Israel. OTOH, Hallel might not have been so well-known, because they just followed along in shul, etc. They did not need to recite for themselves.
This explains the Tosefta's case which is otherwise a bit obscure.