The whole "show us the whistleblower" is one of the most stupid and dishonest aspects of this whole inquiry. It's also such a laughable example of witness tampering and intimidation related to an administration that uses obstruction of justice as SOP.
I work for a Fortune 500 company. No better or worse than any other company, but if a person comes to us and makes a complaint of fraud, or harassment or anything and wants to remain anonymous, they stay anonymous. The case is investigated, and witnesses are questioned.
The underlying relevance of the whistleblower vanishes once other facts and witnesses are discovered. If it is a critical matter and the whistleblower is the only person who can or will support some underlying fact, then there may be a point where the case needs to be "dropped" or the company needs to go all in one what the whistle blower is claiming. (I.e., make a crediblity determination regarding an allegation of sex harassment).
Nobody gets to harass the whistle blower, because it is understood that if the case isn't supportable, it's likely not going to pass the smell test. But more likely, in a case of multiple witnesses, the whistle blower is just listed as another witness, primarily so as not to dissuade whistle blowers in the future.
Sure, once a whistleblower is called as a fact witness post investigation, I think it is fair to find out if they were the whistle blower and what they reported -- that could relate to credibility and be food for cross examination. But at the investigation phase?
Which is all to say that IMO, the impeachable offense is obstruction of justice, and always has been.