You can detune the whole song, if (for example), the whole song was recorded out of tune. It involves some manual hacking of the .dta files, but it's possible. We use it sometimes when authoring a song from the 60s, before electronic tuners.
But this doesn't really help you here, since it a) doesn't give you more than the diatonic scale and b) affects every track, not just vocals.
So Mr. Game, is your professional advise that I simply make it as close as I can, taking this into account and engage the leniency feature?
You guys are a lot more experienced at this than I am, and honestly I'd rather not make this more complicated than it already is
That's probably your best shot. It's probably worthwhile to give it a playtest to see what people think.