Quick impeachment/pardon question

Nobody has to rec this diary, its only for discussion. I would like to hear from dkos constitutional legal beagles please. Here is my question:

Say Trump, tomorrow, gets a hair up his ass and pardons all of the insurrectionists from federal charges. He can do that if he wants to, its well within precedent (IANAL, but I have heard several examples that would be correlaries to this in history). And then, he is impeached in the House and is facing down conviction in the Senate. Would that invalidate the blanket pardon?

I only ask because of this specific clause about the presidential pardon powers 

The Constitution gives the president the power “to grant Reprieves and Pardons for Offenses against the United States, except in Cases of impeachment

Does the “except in cases of impeachment” refer only to the president’s ability to pardon himself when he is imperiled by impeachment? Or does it refer to all circumstances surrounding the impeachment, up to and including participants in the actions that lead up to said impeachment? 

Any thoughts?