Don't forget, Obama can also pardon her for all crimes.
I'm not entirely sure he can for unspecified and unconvicted crimes.
He probably can. Ford famously pardoned Nixon for any federal crimes Nixon "committed, or may have committed." True, that blanket, may-have-committed pardon was never tested in court, but one big reason why not is that federal prosecutors guessed that the pardon was valid.
I'd LOVE it if Obama were to pardon any of the IRS folks in the near future (as opposed to doing so as he leaves office, a la Bill Clinton with Susan McDougal, Marc "Eric Holder Is My Bagman" Rich and countless others). Such a pardon would be a blatant coverup effort and result in instant impeachment. Even if the Senate failed to convict, I'm guessing that every Democrat acquittal vote would be a huge political millstone around that Democrat's neck.
In addition, anyone thus "prospectively pardoned" CAN be compelled to testify, on pain of contempt, because he or she no longer faces any federal crimes for whatever they might say. (I suppose there may be state laws at issue.)