Even if Trump and Pence died, the GOP would never willingly nominate Weld. Maybe if he somehow got all of Epstein's dirt and used it to extort his way to the nomination.
Would they have another choice ?
If Trump places first and Weld second in most primaries, then Trump and Pence die ... why would another candidate than Weld be the legitimate nominee ?
If a candidate died, most of their pledged delegates would be released and become unpledged, just as if they had dropped out (this is why many candidates "suspend" their campaigns rather than actually dropping out). The GOP currently has a rule that you have to win at least five states to have your name placed into nomination at the convention, a bar Weld is unlikely to ever clear, meaning none of the delegates he "won" would actually be pledged to him. Should they reach the convention without any living candidate clearing the five state threshold, I imagine the convention rules committee would simply vote to change the rules and allow other names to be entered. They would probably do the same even if Weld somehow won his prerequisite five states.
Ultimately, if a majority of delegates and a majority of the rules committee wants to do something, no one can stop them.