Why did the House of Representatives not challenge President Trump's claims of executive privilege and/or immunity during the House impeachment proceedings?

President Trump's former chief of staff, General John Kelly has reportedly said, "I believe John Bolton" and suggests Bolton should testify, saying, "If there are people that could contribute to this, either innocence or guilt, I think they should be heard." Do you agree with General Kelly that they should be heard?

The Constitution does not specify the standard of proof to be used in trials of impeachment, and the Senate has not adopted a uniform standard by rule, thus, the standard of proof is arguably a question for each individual Senator. In the Clinton trial and now with President Trump, it appears that Republicans and Democrats apply different standards depending on whether the President is a member of their party. What standard of proof should be used in trials of impeachment--preponderance of the evidence, clear and convincing, beyond a reasonable doubt--and why?

Is it true that in these proceedings that the Chief Justice can rule on the issue of productions of exhibits and the testimony of witnesses over the objection of either the managers or the President's counsel? Would a determination by the Chief Justice be subject to judicial review?

The House managers have argued aggressively that the President's actions contravened U.S. foreign policy. Isn't it the President's place--certainly more than the place for career civil servants--to conduct foreign policy?