According to the US Constitution this is the Presidential Order of Succession.
VP, Speaker of the House, President pro Tempore of the Senate, SOS, etc.

Let’s say JB is arrested tomorrow. How would the SOH not be installed on Jan 20? If NP’s arrested too does Grassley become POTUS?

Does Martial Law suspend the USC? Does FEMA control suspend the USC?

Let’s talk about the scenarios.