Drifting keys: I've come to the conclusion that upward drift is more likely in lively up-tempo songs, and downward in miserable slow songs. If you're singing a capella, this is likely to go un-noticed unless you check your start and finish notes against an instrument, and unless the drift is massive, i.e more than a semi-tone, you'll get away with it.
I do test myself sometimes by checking my start and finish notes against the piano. (and beat myself up if they're not the same!)
I did once compliment a singer on her being able to keep her key after a long spell of a capella, before she came back in on her fiddle: turns out she'd being very quietly plucking her fiddle in her ear to keep herself on key!