Keep going with the bold. It says ant matters that may arise directly from the investigation into any links and/or coordination between Russian government and individuals associated with the campaign of President Donald Trump. That does not give Mueller the power to go on a fishing expedition. That means those matters have to be in the same scope and arise directly from the initial investigation.Sessions made Rodenstein the acting AG by recusing and that gave him the full powers of the AG office, not your made up limitations. He gave Mueller the power to prosecute "any links and/or coordination between Russian government and individuals associated with the campaign of President Donald Trump, and any matters that arose or may arise directly from the investigation" His case is not getting thrown out and if it did it would just get handed off to the FBI and he's right back in prison.
If Mueller tries to indict Trump on something from decades ago that has nothing to do with the '16 election, Russia collusion and/or obstruction of justice then Trump has every right to justifiably fire him.