Turns out, from the discussions over in Season 160.1 and the experiments that I have done since, that what I need to do, once I've finished with the audio, is to export the edited movie to a QT.mov and then export
that movie to a QT.mp4. If I leave it (or re-export it) as a QT.movie, the exported product is some 450%-500% larger than the original.
If I go direct to .mp4, the upload ends up looking really pixellated and grainy. Exporting the bloated exported product from QT.mov to QT.mp4 reduces the file size to approx that of the original, w/o degrading the video quality
of the upload. The .mp4 as it appears on my computer is still really pixellated & grainy, but the video quality in the uploaded .mp4 is just fine.
From then on, the upload is only a matter of a few minutes.
So, those steps in full:
1) Original file recorded in QT (with size, say, of 47MB)
2) Edited original file exported from 'Movie to QT Movie'
OR file exported from iMovie via QT (size increases to 240MB)
3) Exported (240MB) file exported (again) from 'Movie to MPEG-4' (size reduces to 14MB)
4) MPEG-4 output looks grainy/pixellated. Uploaded MPEG-4 looks fine.
If I miss out step 2, however, the uploaded file looks horrible. Still sounds horrible of course, whatever steps I take, but that's the ol' GIGO at work