Is guest right as a custom of conduct totally obliterated now? I mean, Tywin basically paved the way of fearing even “safe” events in the contemporary era, but are there ways to rebuild it?

It’s severely damaged, but it depends on how society at large reacts. If we look at the Northern lords in Winterfell, or the “put up your steel, ser! Are you a Corbray or a Frey? We are guests here,” from Bronze Yohn in the Vale, it seems like the reaction is to recoil in horror and reinforce the custom…by hanging the Freys en masse if necessary.