More interesting autopsy details from John Dowd, Trump's former lawyer during the Mueller investigation for a period of time. Mueller was apparently perfectly understanding of Trump's Twitter outrage; there was a political side to the investigation as much as there was a legal one, and Mueller understood this. The only concern Mueller apparently had was that Trump's comments would act as a message to potential witnesses not to cooperate, but according to Dowd, he was reassured and given full access to all witnesses and it never came up again.
Mueller's appointment and the actions of Rosenstein are apparently a complete mystery. Jeff Sessions was in the oval office with his chief of staff, talking to Trump, when they were all informed Mueller had been appointed, and Sessions was, according to Dowd, mortified. Trump ripped into him, asking how this could happen and how could he not have known? Sessions' chief of staff marched into Rosenstein's office to get answers and Rosenstein was allegedly very hat-in-hand, asking, "Are they going to fire me"? Dowd points out that the investigation never specified a crime; instead, it was a counter-intelligence investigation. Such an investigation is suited to serve the president; it's not an adversarial matter. For that, you specify a crime that requires a special counsel investigation.
The appointment of Mueller by Rosenstein was a complete surprise, and how it came to be is shrouded in mystery.
About the firing of Mueller, and whether that was ever a consideration, Dowd calls it absurd, stating, "Why would you want to do the investigation all over again when you're fully cooperating?" The intent from Trump's legal team at the time was to try and expedite the investigation as much as possible to get it concluded, and there was frustration that it took as long as it did.
If you have an opportunity, John Dowd has a two-part interview with Byron York where he talks about all this, and it's all very, very interesting.