HaLeivi,
The distinction between the Sanhedrin and post-Sanhedrin is not “invented,” it is the meaning of the posuk. Look up the Rambam’s introduction to Mishnah Torah.
Yiftach and Shmuel were each the head of the Bais Din HaGadol in their days.
After the close of the Talmud every town and city is governed, in matters of halacha by its own Bais Din.
I am not saying that there isn’t an inyan of Yiftach B’doro even as it pertains to the individual Bais Din or Posek, but that cannot create binding decisions on the entirety of Klal Yisroel.
I do agree with you, however, that the realm of psak halacha is signficantly larger than people sometimes think it is and the items you raised are certainly within the realm of psak.