Re: Can someone with more knowledge on the subject answer me this:
"With respect, winning by the rules is what having a mandate is defined as."
That may be how it works in a parliamentary system, but in the U.S., where power is split between three separate organizations (President, Senate, and House of Representatives; I'm skipping the oversight power of the Supreme Court for the moment), a mandate is a psychological attribute dependent upon the margin of victory.
Despite his win, Trump's popular vote loss tends to offset any "mandate" claims.
Honestly, the last president who had a clear mandate was Nixon (in my opinion of course; your candidate's mileage may vary). And he had to resign to avoid impeachment. So even a "mandate" won't shield you from your actions.