1) DO the members of Fleetwood Mac NEED a solo career anymore? Why?
Absolutely! Especially since FM releases albums on a much more rare occasion than ever. If the individual writers want to release an album on their own, apart from the sound or confines of FM I think it is essential.
2) Do you think the band wasted time that they can't get back due to their solo careers?
I think that is very subjective. One might say that they could have written so much more great music together had it not been for their solo careers. Then again one could say they wouldn't have even survived if they did not venture into solo careers...time away from each other to vent their own creativities. With this ear I can hear some pretty amazing solo songs from all three members of FM.....I would hardly call it wasted! 
3) Are the "sum of the parts" better?
Without question....YES! What one lacks the other seems to have. One's weakness is the other's strength.
4) Which, if any, ex-members would you NOT be just horrendously offended by if they rejoined the band? (not counting Christine)
First and foremost Rick Vito. I think he would fit like a glove with the present lineup of FM. Danny Kirwan and Billy Burnette would be welcome too, to a lesser degree. It'd be "nice" to see Bob Welch back, but I somehow see CONFLICT in the mix....not just for personal reasons, but musically too; especially with Lindsey. Ahhhh Peter Green.....I think he's just fine where he is now!!! 
Joe