I just found out through a fluke that A Quiet Exit IS NOT required to unlock Mission 46, and therefore we do not need to spend time building the max bond with Quiet and then killing her off.

It may have something to do with a combination of total completed side ops and missions.

Yep. I got 46 to appear first in my first playthrough.

The thing is though that mission 45 is completed way faster than 46 (haven't tested this, but it seems pretty obvious). So it may still be worth pursuing 45 anyway.

i unlocked mission 46 by listening to all the yellow tapes... wouldnt show up for the longest time... so i quickly started each tape for 1 second and it unlocked

you can also redo old missions to trigger MB cutscenes

Who doesn't listen to tapes as soon as possible? Sacrileges!