I'd almost bet on Trump being an unwilling participant to any major collusion, but Pence is competent enough to have known enough of what was going on to be out of reach.

I think Trump really believes he won fair and square, and buys into the bullshit spewed by the sycophants around him. Those people are the ones who conspired and colluded, or willfully turned a blind eye to it, while Trump, disconnected from reality, is mostly manipulated. At least on this. It's more than likely he has committed plenty of felonies before that could easily be used to force impeachment, even a prison sentence, but Pence, Jr., Kush, Ivanka, Flynn, Manafort, Gates, etc, etc colluded with Russia in some way.

Basically, the Trump Campaign, and Administration, colluded, but Trump himself may not have.