I wanted to bring down something from the Hesed L'Avraham (Rav Avraham Azulai) that is tangential to the discussion.
Rav Azulai explains that there are two major parts to Torah (something echoed regarding the soul in the opening chapters of the Tanya (chapter 3 specifically)) there is the part of the Torah that relates to the world of Atziluth, on which level that which dwells in Atziluth can hardly be said to be properly differentiated from HaShem, and so in this world everything is still undivided. The other part of the Torah is the part that corresponds to the three lower worlds of Beriyah, Yetzirah, and Asiyah.
The higher part of the Torah is beyond our ability to flesh out and add to, since it is all united and revealed in the higher world of Atziluth. The only way to reveal new things in this part of Torah is through prophecy or a Bat Kol, a voice from on high. This is the Torah that is still rooted in the upper world.
The lower part of the Torah can be fleshed out and determined through a gathering of all the Talmidei Hochamim, Torah scholars, in a particular generation. This is the Torah that is determined not according to the dictates of Heaven, but according to the majority, the rov, of the learned Rabbis.
The explanation of this difference is that only in these lower worlds is there a concept of multiplicity, and so down here, the majority rules and it makes sense that the majority rule. However, in the Torah of Atziluth, the majority is meaningless as the inhabitants of this world are one undifferentiated whole.
To tie this in to what Chabakuk Elisha was saying, minhag is a holy transmission from one generation of Judaism to the next that is thoroughly and wholely involved in actions, in the world of Asiyah. As it is Torah from the level of Asiyah, the majority of the Jewish nation rules in this respect, and so what we do becomes part of the Torah of these lower worlds where the choices and actions of the informed majority have such an impact. That's one of the reasons, perhaps, why a minhag that is a universal minhag (minhag Yisrael) is considered Torah.