If the White House counsel is scared of Trump firing Mueller, it would prob. work to our advantage
if he did. Lets face it, Mueller or no Mueller, the FBI is closing in on Trump and if they take the head off of a snake that can grow a new head each time one is cut off, Trump just looks guilty as hell if he follows through and all hell breaks lose as the snake grows a new head. Only Rosenstein can fire him, and Rosenstein has sworn not to do it without a really good reason.
McGhan is probably somewhat loyal to Trump as they have had ties for years. He isn't advising Trump not to fire Mueller because he doesn't like Trump, he is doing so because it's good legal advice.
Either way it looks like Trump is in hot water. It must be checkmate if he is so desperate he would consider firing Rosenstein then put someone in place who would fire Mueller and create a crisis of epic proportions that would blow up in his face. And if the FBI has built their case he still goes down but even harder.