the concept of over/extra system skill/abilities, where a player can overwrite limitation set by the game system, does get expanded on in the later arcs, especially in Alicization, though other examples also exists, such as Kirito's ability to execute multiple sword skills in tandem in Calibur (Kawahara needs to come up with a better explanation for that one though IMO
), the end of the ALO arc etc, with the end product being somewhat similar to the Incarnate System in Accel World by Alicization, where Kirito can literally alter things in the VR world through thoughts alone.
Now Underworld is a different entity from Kayaba's SAO, but Underworld is based partially on Kayaba's work, and IIRC is based off of the Seed as well, so it's not really much of a stretch to say that there's something in the Cardinal system in SAO which allows for people to break the system/rule so to speak, esp. since the entire point of Underworld is to try to find ways for the fluctlights to break the rule. This remains a speculation however, as the inner workings of these systems remains ambiguous.
Now about Asuna and Kirito's survival at the end of SAO, that actually have nothing to do with overwriting the system or any sheer willpower. As Kayaba said to them during their post-fight meeting when they asked about the players that died, he confirmed that he specifically brought those two there because he wanted to talk to them one last time. (Though he did not tell them they weren't going to die, one last lol-troll by him I guess
).