Are there places I can nurse my baby without too much disturbance?

Yes.

There is no place to sit in a bathroom and nurse a baby at the museum. There some benches outside of the toilets. It is not necessarily quite some days.

There are plenty of bathrooms throughout with the exception of the concourse which is below grade. When you exit the elevator at the bottom, there is a bathroom to the right... no other bathrooms are available until you work your way back to the main level. This may take a while so consider that when enjoying that section of the museum.

The Bathroom is clean but i don't recall a sitting place in there. Ni

Yes on every floor 😀