Is the harp person Joanna Newsom? Agree with you on that front, but disagree when it comes to Dave Gedge. Missed concert or no (yes, I'll go there) I kind of like his voice, with the harder, almost awkwardly angular vocal delivery underscoring the painful themes of lost or unrequited love in some of the Wedding Present's better songs (see: 'Skindiving', off 'Saturnalia')

As for your other questions, Neko Case always elevates a song for me...'soaring' seems to be the best, albeit clichéd way to describe her voice but there's a fullness to it that feels as though it were being carried across a landscape. Her songs are good to begin with though. An example of a bad singer who doesn't ruin a song might be Leonard Cohen, (see: 'Suzanne') . Do so many Pavement songs work in spite of or because of Malkmus' voice? [For sure, their videos suck].

Frank Sinatra certainly made an ordinary song sublime....'Fly me to the moon/Let me play among the stars' is no great poetry but the words sound golden when they come out of his mouth. Chet Baker also lifts many of the jazz standards.

For a more recent example, Sam Beam from Iron & Wine writes some very non-complex, stripped down acoustic songs but his voice envelops you in its warmth, even when the material has recorded on low-fi equipment in his own home, which I believe was the case with 'The Creek Drank the Cradle'. I'd argue that may qualify as lifing the material but I'm guessing what you're looking for are examples where the source material is bad in the first place.

Will keep pondering on that one.