physical flaws (mordred, noctis etc..)

a quick question:

noctis had a sword, which if used on him would kill him, in any other way, he would be immortal, now imagine with me, sunny went into a nightmare with it, and used it on his past self, destroying him, this means that his present has become deadly again? or that he now has no weaknesses left?

If someone has a flaw, which is an object, that if it is destroyed they die, and that object is left in a nightmare, can they live freely without that flaw?

We should think about this, because we know that Mordred's flaw is also something he can take from his body, probably a mirror, or something reflective, and we have no idea where he is.

edit: I know that Noctis didn't have a physical defect, I'm talking about his sword as if it were a second defect