Only reason Tressel was fired is that he signed the NCAA compliance letter stating he knew of no violations. If he hadn't signed that and then they found the email that he did know about it -- that is what screwed him over.
If Harbaugh did the same -- He should most definitely be fired. If it is true what is being reported about the ticket purchasing with a personal credit card and paying people via public venmo -- I can't fathom Harbaugh knew about it, as it was so blatantly out in the open.
That being said -- even if Harbaugh didn't know, he is still responsible for his staff, so he could be on the hook. All depends on what we find out actually happened once the investigation is done. He could still end up being fired even if he didn't know about it.