I can agree with what you said. It's true that Federer would have fewer Slams if he was the same age as Nadal, Djokovic, and Murray. But then, so would they! Nadal would have no Hardcourt Slams, Djokovic would have no Wimbledon or French Open, and Murray would have 0 Slams.
dunno about the last point. Murray basically faced peak Nadal and Djokovic his entire career. And had 6 wins over Federer from 06-09 generally Fed's peak is generally regarded as Wim 03-Oz 10. Murray might well have beaten Federer and even if he didn't Djokovic or Nadal might have beaten him and Nadal beaten them. I'd still give Murray 1 or 2 slams if they were all the same age. He continually improved (until US 2013) and was striving for success. He may well have found a way.