yitayningwut: Certainly there is “halachic truth.” But when we refer to “halachic truth” we refer to a halacha being unalterably established either by the Sanhedrin or through the compilation of the gemarah. After that, all is fair game (though that might be a poor way of putting it since any good posek understands the different levels of deference that must be given to a ShuT of Rishonim, Achronim, Mishnah Torah, Tur, Shulchan Aruch, Aruch Hashulchan, Chidushim on Shas, ect.).
I cannot agree when you say, “When there is a machlokes Rashi and Tosafos; Rambam and Ra’avad; Shach and Taz; or R’ Moshe and R’ Yaakov, one is right and one is wrong. End of story.” That is simply not the case.
True, one may be accepted in practice while one is not, but that doesn’t mean that one no longer exists except in the “Bashamayim” realm. Eilu v’eilu tells us that both are correct. Mesorah tells us that we follow the opinion that has become accepted practice. Nevertheless, the fact that one is accpeted and one is not does not mean that under extenuating circumstances, and posek can’t dredge up a daas yachid or unaccepted view and rely on that to develop a psak to be mattir something or vice versa.
There is one “true halacha” in Shamayim, perhaps. Maybe God has a single way that He KNOWS the halacha should be. Here on earth, there is no single true psak (again, expect for rulings of the Sanhedrin and chasima hagemarah). As the Rama to CM 25:1 points out, a posek may, with proper reason, go against an accepted halachic practice and follow a rejected opinion of the rishonim. That rejected opinion is not invalid; it is merely not accepted in practice.