I've been strugging to compose an answer which is shorter and more to the point than my complete musical autobiography :rolleyes: I was so proud of myself for making less-than-infinite opening post on this subject, and then you guys hafta go and ask me all these questions!
I tend to eventually run into trouble when figuring tunes out by ear from zero. But when I'm transcribing recordings of Irish tin-whistle players to see how the do their ornamentation, it's no problem at all for me to edit the generic off-the-web version of the tune to match the variations they play on the recordings.
I also think something might be a little iffy about how I hear my own voice, though. I have no problem pitch matching quite nicely when I'm playing a tin whistle or a recorder (which, for the uninitiated, can take a bit of breath adjustment to get notes in fully in tune). But when I (attempt to) sing, I tend to want to aim too high, and need to put my voice lower than what feels or sounds right to me in order to get the resonant sensation of that I recognize as pitch matching.
Recently i got a good discount on this piece of software: Sing and See
which has been helping me get a better handle on what's going awry when I attempt to match pitches with my voice.
I also have a friend who's had formal vocal training who's going to help me with breath support, because my singing voice is very weak and wavery (after years of being discouraged from singing at all!). Right now it's just a matter of logistics to figure out when this is going to happen.
Gotta go for now, but James, I'm still thinking about your deep questions.