1) Lyrics don't have to make any sense whatsoever in a literal sense - they only have to work when sung. Ever heard "Land of a Thousand Dances"? "Na, na na na na, na na na na, na na na, na na na" is sheer brilliance in the context of that song. As far as vocal quality, I prefer melodic singing, but there's no denying the greatness of people like Johan Hegg, Nergal, Phil Anselmo, Zack de la Rocha...or DGN faves like Ogre and Al Jourgensen.
2) My fave band is U2; while The Edge certainly has the technical ability to handle the lead vocals, no one could take Bono's place.
3) Disregard the lyrics? No...but all sides would be better served by doing something at least a little bit different musically.
4) Yes...and yes.
...well, that's what I got.