Is a presidential pardon still valid when the president is later impeached specifically related to the actions pardoned? I think if Hillary is impeached there cannot be a presidential pardon.
To answer your question, my understanding of the Constitution is that only impeachment is not subject to Presidential pardon, but I would find it hard to understand how one would impeach Clinton given that she is no longer an employee of the Federal Government. I'm not quite sure how it works to go after a previous employee. Maybe a constitutional lawyer can weigh in.
I think if Hillary is impeached there cannot be a presidential pardon.