The connection between 2005 tax returns with 2016 collusion is?
It is not about tax returns or collusion. Mueller is trying to squeeze Manafort to testify to something/anything about President Trump. But even if Manafort turns, he has nothing to say anyway.
Most of the talking heads I have seen commenting on this, say that Mueller wants Manafort to come up with something/anything about Trump, even if he has to make it up. Mueller doesn't care.
I hope Rosenstein and Sessions(wherever he is...?) are proud of themselves for ruining this man's life for absolutely nothing and no reason. That is how fanatical the Left is. Since they can't get to Trump, they will settle for any warm body to serve as Trump's proxy. The whole thing is like a scene from the old U.S.S.R.