Your statement ("modern executive interpretations that 'due process' in the law simply means any established process") is not consistent with Holder's statement, as described by the very title of the article you linked to: ("Attorney General Holder: Due Process Doesn't Necessarily Mean a Courtroom.")

Due Process does not require a courtroom. That does not mean that "any established process" will suffice. The amount of process due depends on the context. The open-endedness of the concept is embedded in the phrase itself: the word "due" means "warranted" or "appropriate."

I don't think there's a Supreme Court case specifically involving police officers, but the general idea that public employees have protections against suspension is well-established: http://www.huizenga.nova.edu/course-materials/6240/cases/Con....