According to an article in the Washington Post, Barry Goldwater retired as a Major General in the U.S. Air Force Reserve in 1967. That was well after he was elected to the Senate and had made an unsuccessful run for the presidency.
I don’t think the army of which Washington was general of existed by the time he was made president.
I think the term you are looking for is coup d’état. If successful it automatically becomes legal.
Of course it did. General Washington was commander of the Army of the United States of America. The fact that it was operating under legislation passed by a body which existed from a previous constitution is of no consequence.
One of the unfortunate confusions about our early history is assuming that anything done before the adoption of the Constitution of 1787 was somehow not done by our current country. This, of course, is wrong; the country was born in July of 1776, which we celebrated the bicentennial anniversary of in 1976. 
Does that mean that he can’t receive any federal monies whatsoever or simply that he can’t receive any additional money from the US or any state government for being President?
It looks to me like he can’t receive any at all, else why would the limitation extend to the individual states? So the President can’t receive food stamps, he can’t take a job as a plumbing inspector, and he can’t be awarded a Gugenheim grant for his artwork.
Or assassination. That is, only if you think the VP would be a better commander. :dubious: