So, forgive a poor foreigner's ignorance, but how does this work - the Senate rules that impeachment is constitutional, and McConnell votes to acquit because impeachment is unconstitutional? Can a juror vote according to his own personal version of the law? Can a murderer walk free if a juror feels that killing people is not actually illegal?
McConnell was not in charge of the Senate for the trial. The Democrats were. So there was a trial because the Democrats voted for it. They thought it was constitutional. McConnell is now just another Republican senator who can vote for guilt or for acquittal. He felt as others did that trying a civilian isn't constitutional. That only president can be impeached and tried for removal from office. Since Trump was already a civilian at the time of the trial, the trial is moot. So McConnell voted for acquittal.
What would have been interesting is if the Senate did find Trump guilty, and ruled that he could no longer run for president or another federal office. Then there would have been another trial where Trump would have sued claiming his trial unconstitutional. The whole case would have gone to the Supreme Court. They would determine the Constitutionality of the whole process. If you recall, the Chief Justice of the Supreme Court, who is supposed to be the judge of all impeachments of presidents, refused to sit as judge in the Senate for this impeachment. He sat as judge for Trump's first impeachment trial. So the Democrats assigned a Republican senator who already stated before the trial that he thought Trump was guilty, certainly an unfair situation when a judge has pre-judged the outcome. But the main point is that the Chief Justice bowed out because he understood the constitution did not allow him to be the judge because Trump wasn’t president at the time of the trial. So it's very possible they could have ruled the whole impeachment invalid.
Regarding personal views, jurors are supposed to follow the law. However, when juries are selected, there are interviews of prospective jurors where these questions are asked before the trial. If this issue comes up, they would be excluded from being selected as a juror.