In my experience (mostly on Ultra-Long TPAC flights) United's flight attendants will bend over backwards to help parents with infants, and try and make travel on board as pleasant as possible for any infants and children on board.
I've been critical of United's FAs on occasion on this forum, but this is one area where I've witnessed nothing but care and compassion from the FAs.

I suggest NOT to put any FA in the position of having to choose between accommodating a passenger with an infant and their employer's explicit policy.
Just put yourself in the shoes of the FA: Would you feel comfortable bending the rules for a passenger, but at the same time risk the airline and its pilot getting in serious trouble over it?