Chronis is right, Smapti is right.
If the constitution says “do this” and does not say “you can do that”, then “this” is what happens.
The constitution makes no provision for elongating the president’s term - it explicitly says when it ends. So on Jan 20th, Wallace is no longer president.
So who is president? If Truman is certified, then all that is required for him to become president is for him to be sworn in. Recall Johnson was sworn in by a judge on the way back from Dallas in 1963. It does not have to be a grand ceremony complete with crowd and parade. All Truman would have to do is find someone to administer the oath of office. Some debate would ensue whether the oath is necessary or whether he automatically assumes power.
And, as mentioned, failing that, or if Truman’s election is not certified - then there are explicit instructions over who are next in line to be president. In 1945 this was Secretary of State. There are no explicit constitutional rules that the SoS stops being so because the president’s term expired, so SoS it is.
I guess the next question is - does the president have the authority to cancel a meeting of congress? How fixed is that Jan. 6 certification, what happens if it is missed for some reason? (I don’t recall, did Biden’s final certification creep into Jan 7th with all the delays?)
And, like every constitutional crisis, the question is how blindly others would obey what could be perceived as illegal orders or inappropriate use of powers? If the president says “stop congress meeting” and a quorum of congress, the police, and the military say “no” then- congress will meet.
Wikipedia:
Had the president died, resigned, been removed from office or been disabled during one of these vacancies, the secretary of state would have become the acting president. Although such circumstances never arose, President Woodrow Wilson apparently drew up a plan whereby, - given the turmoil of World War I - if his Republican opponent Charles Evans Hughes had won the 1916 election, then Wilson would have dismissed his secretary of state, Robert Lansing, and recess-appointed Hughes to the post before Wilson and Vice President Thomas R. Marshall both resigned, thus allowing President-elect Hughes to serve as acting president until his March 4, 1917 inauguration. Wilson’s narrow victory over Hughes rendered the plan moot.[20][21]
Also of note is that 1940 Republican presidential nominee Wendell Willkie and vice presidential nominee Charles L. McNary both died in 1944 (October 8, and February 25, respectively), the first (and as of 2021 only) time both members of a major-party presidential ticket died during the term for which they sought election. Had they been elected, Willkie’s death would have resulted in the secretary of state becoming acting president for the remainder of the term ending on January 20, 1945.