Can Mitch just send it back to Nancy with a note to the effect, “Next time admit the dog ate your homework.”?
Right now he has nothing to send back. Once he gets the charges, the rules spell out the sequence. Trump is summonsed and ordered to answer, in writing. And so on.
There is lots of flexibility how the senate handles this and the grounds for decision. One of the grounds could be that the articles do not state an impeachable offense. I think Clinton made that argument, but the senate didn't vote on that.
You missed the point. He can’t “send anything back” until he first GETS it from the House.