I don't know how the system works.
I predict Mr Trump will end the Russia Investigation. Despite all the peremptory threats from Republican Senators I also predict neither Sen McConnell nor Rep Ryan will do anything to either stop the action or react to it.
Now this is what I think will happen. All the research and evidence will be archived or destroyed. I don't know if SP Mueller could write a report from the position of a discharged SP. I don't know what value it would have. It may still not matter since it is not likely Democrats would win a majority in the Senate.
Mr Trump (as president) has become a person above the law because of his singular position as president.
The 2020 general election is in jeopardy. If Mr Trump loses the 2020, no Trump supporter will believe the results are valid. This would be a crippling blow to the foundations of government. Remember there are a lot of extremist nuts in Congress.
just some rambling thoughts