If your threat actor is omnipotent, you’re going to have a very hard time designing a coherent threat model against them. Neither Signal nor PGP nor hiding under the bedsheets will protect you from an adversary capable of directly recording your screen.

(But note: absent something like that, it in fact is possible for Signal to prove that you’re communicating with the same identity you initiated with. That’s the whole point of authenticated key agreement, which is not something that PGP can natively perform.)