Hello Herb,
Before I bought my Melodyne Editor DNA, I went through lesser versions of Melodyne, so my referral to Melodyne Bridge relates to those versions. (Cre8 and Studio)
Most audio transposing tools I use (not just Melodyne) offer formant control. While perhaps useable for gentle vocal fixes within a few cents, it makes transposition results far, far worse once serious semitone distances are involved. Having tested in detail what it can offer long ago, I avoid it like ebola. If it works for you for vocal autotuning and subtle harmonies (something I don't do, so I can't comment on ... nice song, congratulations) - wonderful, at least you have a use for it.
Yes...as long as you use anything within it's designed purposes, things work well. I don't think there is a program yet that can transpose vocals half an octave in either direction and have them sound natural. Currently, even the best we have now imparts very noticeable artifacts. Even in the demo video's for melodyne, they rush past those female to male vocal changes with melodyne because to the discerning ear, the artifacts are glaring.
The better way to work would be to get a singer who sings in the range needed rather than to try to move some other singer's track that far.
Even Band in a box, with it's sampled lines, uses parts recorded in various keys so that the transposition from it's native key to the key you select is only a few semi-tones at most. I have even started the habit of checking the native key of certain parts before I select my song's key, for that very reason so that BiaB doesn't have to transpose at all. And there are times when you can hear artifacts in the music from the key change.
That doesn't make Biab or Melodyne or any other program a piece of junk if I'm using it out of it's designed purposes.....it simply means that I need to work to find a better way to accomplish my intended purposes with music.
And yes, my intention with Melodyne is to use it manually, not in auto mode, so I can control precisely what it does. And most of the time it's used to tune or edit notes that are 99% there already or the timing, or other things like vibrato, or levels, that are well within it's design characteristics and abilities to edit without audible artifacts. And it does that job exceedingly well.