I have to say the "not singing" part bugged me more. Singing doesn't require any mechanical devices, while playing a guitar does (the guitar itself, of course). What are your thoughts?
No, it was Mark Campbell that sang Johnny B Goode. To me, it just never made sense. I can understand having someone else play the guitar for him, especially if he wasn't very good at it (MJF is actually an excellent guitarist!) - but to have someone sing in his place is simply wrong!
I would have to say singing. But since I know that MJF can play guitar that bugs me a little bit too. It doesn't really take away from the movie though. I wonder if the Bobs ever asked MJF if he wanted to actually sing "Johnny B Goode" though.
Right, exactly, we know Michael J Fox can play the guitar well. That's not as jarring, though, as there are lots of things characters can do in movies and TV shows - that the actors themselves can't do.
If Marty McFly's spirit was the enter the body of another person, he would be able to play the guitar the same way - and it would sound the same. With a voice, though, that's different. It's a thing that a person naturally has. If you're going to do that, you might as well just have Mark Campbell voice all of MJF's lines in the movie - and have MJF only be there so that Marty McFly can look like him (but not sound like him). I'm sure many would argue that MJF's voice is just as much a physical attribute to him as his appearances are.
BTW, it would be more understandable in cases where MJF was playing Marlene. Some people think it was someone else who voiced Marlene's lines - but concensus seems to have it that MJF himself did altere his voice to make it sound feminine.