You can't have one without the other. If you feel you have no chance breaking serve then holding serve becomes more difficult - its all in the mind. And no one breaks serve by standing on the fence. Not sure about the success on return comment, Murray has lost all of his big matches in the 4 slams bar Warwrinka, and he almost came unstuck in that as well. Murray may return well against the top 30 players but he has to return better against the top 10 opponents in the big matches (i.e. slams not ATP events which are best of 3 sets)
Has Murray had any injuries or surgery on his right wrist?
Murray is statistically the best first serve returner on tour. He is also statistically the 2nd best 2nd serve returner, 2nd best in terms of return games won, and 2nd best in terms of breakpoints converted. Only Rafael Nadal is ahead of him in those categories and Nadal's return stats get a boost from the clay court swing. Murray was ahead of everyone on all 4 return of serve categories this year until the clay court swing.
And yes, you can have one without the other. Karlovic is evidence of that. He can't return to save his life, but he holds serve more than anyone else on tour. Murray broke Federer's serve several times that day. The problem was that he kept getting broken because his serve is inconsistent and he allowed Federer to dictate and was too central with his groundstrokes. He didn't find Federer's backhand often enough.
No, he hasn't had surgery as far as I'm aware. Just an extended period of rest to allow the wrist to heal after he injured it in Hamburg 2007.