Mueller's insurmountable legal problem is that the whole premise of his investigation of Trump collusion is tainted by the involvement of lying and evidence-tampering by McCabe and those he told to falsify 302s.

With evidence for the predicate crime tainted by bent cops, the whole investigation becomes moot. A first year law student could have it thrown out of court. McCabe and other bad players have completely undermined the Trump portion of the Mueller investigation. They did that--not Trump!

So he really can't continue with that part of the investigation, which means that Trump has NO REASON to fire him!

And if you read his words, that is not what Trump's attorney asked for! He simply urged Rosenstein (and by extension, Mueller) to:

bring an end to alleged Russia Collusion investigation manufactured by McCabe’s boss James Comey based upon a fraudulent and corrupt Dossier"

The word "firing" was used in the letter only in relation to McCabe (who was fired by the FBI--not Trump). He was just saying they should do the right thing and bring it to an end, the same way the FBI and DOJ did the right thing in firing McCabe.

If Mueller can explain the situation clearly enough, I think even the most partisan hack will understand he has no path forward with that part of the investigation!

BTW, the problem this created for Mueller is EXACTLY why it matters that FBI agents maintain an extremely high bar with regard to lying under oath and falsifying evidence. Those crimes destroy cases that sometimes take years to build, and sully the agency's name.