The surging economy will counterbalance the historical advantage to the out party. Most likely we lose 0 to 10 seats. On a really bad day, it might go as high as 20. Meanwhile, we gain 1 to 3 in the Senate.
The President has to make the public believe that the economic gains are a result of his deregulation and tax cuts then the Republicans might retain majority.
The investigation into collusion among the democrats and our intelligence agencies to steal the election might help but I think that the lag time between the opening of the investigation and the guilty pleas will more likely affect 2020.
If the dems take the House Trump will be impeached. Not for legal but rather political reasons. The President will not interview well if deposed however. There just needs to be enough Republicans in the Senate to not convict him. Then the question is will he have the political capital to win in 2020. If the Senate fails us will the SCOTUS go along with demanding a President resign for political reasons rather than “high crimes or misdemeanors”.