Is there an Affirmative Defense to Obstruction of Justice?

One the face of it, I would say that there are 3 counts of Attempted Obstruction of Justice from the Mueller account:
1. Trump asking Session to reverse his recusal.
2. Trump asking McGahn to fire Mueller.  (McGahn did not have the authority to fire Mueller but Trump didn't know that).
3. Trump directing McGahn to create documents that would say that Trump didn't ask him to fire Mueller.
The other accusations in my opinion do not rise to the level of attempted obstruction.  For example, asking Lewandowski to fire Mueller or to fire Sessions.  This was a bad joke on Trump's part, because Lewandowski wasn't even a government employee and had no authority at all.

That said, I don't think these would justify his removal.  Instead Congress should just censure him and move on.

The affirmative defenses to the charges are:
1. The right to resist an unjust investigation.  This is analogous to the right to resist an unlawful arrest.  The burden would be on the defendant to prove: a) that there is such a right, and b) that the investigation is unjust.  Trump is trying to do this by have Barr and others investigate what lead up to the FISA warrants and the espionage on his campaign.  Trump hasn't made a coherent or persuasive argument that there is a right to resist an unjust investigation, but I think he or someone else, like Guiliani or Sekulow, could.  
If Trump doesn't have such a right, then he would have a duty to comply with the investigation even if it was unfair.

2. Executive privilege to resist an investigation.  The president does have extensive executive privileges; however, he hasn't asserted it until recently, by telling people not to appear at the Congressional investigation..  He was cooperating with Mueller's investigation.  It seems unfair to criticize Trump for cooperating before, but you have to be consistent.  So I don't think executive privilege was properly asserted here.

So I think the only thing that will help Trump is a persuasive argument that there is a right to resist an unjust investigation.  I haven't seen it, but I haven't looked that hard yet.

This is related to the idea of limits of Presidential power.  Do we agree that no one is above the law, not even the president?  Or is the president presumably privileged to do anything, including shooting a man in broad daylight on Fifth Avenue, and the only limitation on his authority is the ability of Congress to impeach him?  That is the debate we as a nation are having, or should be having.

Update:
There is a 3rd affirmative defense, closely related to the other two.
3. The President cannot be charged with obstruction of justice for lawfully exercising his constitutional power.  This was stated by Alan Dershowitz.  I don't think Dershowitz goes this far, but under this theory, the President can fire anyone at anytime for any reason, even the special counsel.

Did Nixon lawfully fired Special Prosecutor Archibald Cox?  The Articles of Impeachment against Nixon (which did not pass the full House) do not mention his firing Cox.  It only says: "4. interfering or endeavouring to interfere with the conduct of investigations by the Department of Justice of the United States, the Federal Bureau of Investigation, the office of Watergate Special Prosecution Force, and Congressional Committees;", so maybe it is implied there, but it isn't clearly stated.  The investigation against Nixon continued anyways under the new prosecutor Leon Jaworski.

So if Trump could have lawfully fired Mueller, it isn't obstruction for him to try to get Sessions or McGahn to fire him.  That leaves one count of attempted obstruction of justice, which is the claim the Trump told McGahn to create false document of the request to fire Mueller.  And Trump claims that he never told McGahn to fire Mueller. "  Trump tweeted: "I never told then White House Counsel Don McGahn to fire Robert Mueller, even though I had the legal right to do so."  Is McGahn lying or is Trump lying?  Does it matter if Trump did have the right to fire Muller?

So is there any evidence against Trump not related to his firing Comey or attempts to fire Mueller?  Maybe, if you believe Cohen.  But not much is left.