I would have thought the UD is rather impractical for so many reasons. From my experience visiting the UD is much more likely to be restricted than to equivalent seats downstairs, although the ability to do either is still minimal / very short term anyway. So I don't think the nanny is going to really be part of your flight experience.

Wouldn't 2 bassinets be easier rather than 1 with other passengers seated beside you. I would look very carefully at your seat selection and think it through against likely scenarios to see if you have really made the best choice.

I am a litle surprised that infants are allowed to be seated/on laps upstairs from a speedy exit point of view when many other groups are not.