Thanks for your reply. I only addressed the question as to whether a U.S. President can be impeached for acts/omissions committed prior to holding office.
But to address your comment: I think it's futile now to debate properness/likelihoods of convictions, without knowing the results of the Mueller investigation.
Yes. Impeachment is a political remedy so congress gets to determine what is impeachable.
For example, the southern district of New York just tied Trump to illegal campaign financing. This being prior prior to his holding office. The House could raise articles of impeachment just on that, pass it to the senate and the senate would then hold a trial.