"I do solemnly swear (or affirm) that I will faithfully execute the office of President of the United States,
and will to the best of my ability, preserve, protect and defend the Constitution of the United States."
So as you see ;the oath is more than a declaration to protect and defend the Constitution.
I know that the issue of inherent powers is debatable ;but if it is ,then every President including Washington has violated the oath according to your narrow definition .
Inherent in the role of the President is the most important area of national security or "in the national interest" .
Generally recognized by both Congress and SCOTUS is that the president has "special prerogatives" in foreign affairs.
Right or wrong ;SCOTUS decided in 1936 that the Presidential powers was "special and pronounced" in foreign affairs .(US v Curtiss-Wright). Even before that John Marshall wrote that the Presidency was the "sole organ of the nation in its external relations, and its sole representative with foreign nations" .
Before that Jefferson said in 1790: ''The transaction of business with foreign nations is executive altogether. It belongs, then, to the head of that department, except as to such portions of it as are specially submitted to the Senate. Exceptions are to be construed strictly.''
The justice dept. memos cite the sole organ doctrine in their rationale and since there is 2 centuries of prescident behind them it is hard to make the claim that the President was not faithfully executing his office.
Congress can make law and SCOTUS can make decisions but unless you can prove your contention that the memos were written after the fact ,then any prosecution will be in itself ex post facto.