If I do that then my credibility becomes relevant and hence so does evidence that there are problems with it (what you call ad hominem).

For example, you've claimed several times that people should believe you because you are an academic historian. However, you've also demonstrated an unfamiliarity with how academic politics works and with basic aspects of logical argument. This can lead to sevral possible conclusions:

1) You're lying about being an academic historian.

2) You really are an academic historian, but are bad at observation and reasoning, and thus likely to be incompetent at your job.

3) You really are an academic historian who is perfectly willing to lie and commit logical fallacies when it helps advance your argument.

Either way, we no longer have any reason to believe that you claim on your authority.