The OP blurs the case between the Prez & VP being killed or being temporarily incapacitated.
If they’re both dead, the speaker becomes by title the “acting president”. But by law they’re fully the president in all regards until the next regularly scheduled election. As they say in the law biz, it’s a distinction without a difference.
OTOH, if prez or VP are alive, but incapacitated, then everything gets murky. As discussed in this other recent thread:
And equally clearly, there are two different USAs and constitutions and laws in effect. The one that existed up until a couple months ago, and the one that exists now. For any political hypothetical question we need to specify whether we’re dealing with the pre- or post- Jan 2025 version of the USA. Those two eras’ governmental arrangements have little in common now beyond the window dressing and less all the time.