In the very unlikely chance that Pence is President in time for the 2020 election, I could see Trump giving Pence an ultimatum of pardoning him or telling his supporters not to vote for Pence.
Maybe, it's a risky proposition, pardoning Nixon basically cost Ford the following election. It's difficult to foresee how the public would react, I think lean on the side of history and don't pardon him is the safer bet.
Either way, and I understand that injustice should be dealt with, but I think Congress should play it politically, a ruined Trump is an easier defeat than a healthy Pence I think. Not just the Presidency, it's a poison pill for the Senate too. Anything that makes them look bad is ultimately better for Congress, and in this particular instance, America too.
I would love to think Trump's defeat is a sure thing, but I don't, at all. I have underestimated stupidity time and time again with Trump, and Brexit, and other mind bogglingly bad calls the public have made. Have Trump be a shamed husk and lose, it's better for everyone.