Here's progress on this tune, which I'm getting nearly done. Or rather I've put as much time into this as I can bear - working on crazy tedious tunes makes you crazy and tedious eventually.
3:42 is a pretty drastic addition bum-ba-dubba-dubba-bum, but there's been little (and not so little) changes peppered throughout.
As for the mastering, it's mostly just some subtle compression, tonal balance and limiting, although nothing more than a dB chopped off, and hopefully avoiding sounding loudness-war-y.
Also the ending is becoming a separate track after my sister contributed a lovely melody which I'll share in a bit.