In 1876 there was a big mess like this. It ended up by a commission being appointed from both houses and the supreme court. They ended up with a compromise. While Tilden won the popular vote, the EC had 4 contested states and was too close to call. The compromise gave the all 20 contested EC votes and the election to Hayes but gave the Dems an end to reconstruction and a withdrawal of troops from the south.
My guess is that if massive fraud was proven, the SC would declare the election void and send it to the house or to congress as a whole. Either way would likely be a victory for Trump.
