I don't believe there's any way for him to do that. If Sessions was fired or resigned he would be the acting attorney general until a new one was confirmed. After that it's up to whatever that new AG would decide. I'd say it's a pretty good bet that the replacement Trump hires will be on board with firing Mueller.Doesn't rosenstein have the power to make Mueller untouchable if he felt like the new guy would impede?
They may just continue with their current plan, which is to attempt to paint members of the investigation as having conflicts of interest.