The brief description of Trump’s argument above is persuasive. I suspect our current USSC will ultimately agree.
The separation of powers argument is sound, of course, and perhaps it is the easiest argument to make. However, it is maddening that it would even be necessary to make it - since there is nothing wrong with attempting to “overturn” an election by challenging its veracity.
It would be one thing if he attempted to overturn it by force - but simply demanding that evidence of fraud be heard is not a crime. In fact, it is a patriotic duty.