Impeachment is a political process that does not follow the same rules as a criminal proceeding. Removing an elected President by impeachment and Senate conviction is also decided by elected representatives.

It wouldn't change the outcome. If the Senate isn't the jury, who would be? The Constitution doesn't say they have to be a good jury. There's nothing about a retrial.

Just as under the Constitution the Senate has no business weighing in on how the House conducts an impeachment (Art I, Section 2, giving the House the "sole" power of impeachment), the House has no business weighing in on how the Senate conducts an impeachment trial (Art I, Section 3, giving the Senate the "sole" power to try impeachments).

It is conceivable but highly unlikely that one or more Democratic Senators could object to the participation of Republicans who have gone on record against conviction before the trial; for example, they might object that such a Senator cannot truthfully take the required oath of impartiality: "I solemnly swear (or affirm, as the case may be) that in all things appertaining to the trial of the impeachment of [name] now pending, I will do impartial justice according to the Constitution and laws: So help me God." But that isn't going to happen because it would only lead to the Republicans scouring the public record for statements made by Democratic Senators prior to the trial in which they suggest that Trump has committed impeachable offenses -- statements I suspect wouldn't be all that hard to find in some cases.

focus on a single issue to impeach... he can be impeached multiple times and for each, I think, separate incident there will need to be a Senate trial. But I suspect that this is being done mostly for the public to get on board. The thing about this culprit is that he keeps making more cases and proving he has something to hide.