US Presidential succession

You are correct. It is of course Article II, Section 1, Clause 6

But Senator Blount’s impeachment which raised the issue was 1799 and the next presidential succession act (1886) eliminated the SOH and PPTOS from the line completely. As I’ve said, SCOTUS would probably have to get involved and God only knows how they’ll rule on an issue (e.g. that XXVth Amendment may completely overrule II,1,6 and eliminate the “officer” requirement.

Yes, on June 29, 2002, Cheney served briefly as Acting President under the terms of the 25th Amendment: Acting President of the United States - Wikipedia

Another possible solution to the worst-case presidential-succession disaster scenarios: Should governors be added to the Presidential line of succession? - Great Debates - Straight Dope Message Board