Q. Is one allowed to answer amein between mayim achronim and starting benching?

A. It is evident from Shulachan Aruch (O.H. 166: 1) that even after washing netilas yodaim at the beginning of the seuda, you may answer amen before hamotzi, more so after mayim achronim. (hand washing before reciting grace after the meal).
Therefore, Horav Shlomo Miller’s Shlit’a opinion is that you are allowed.

Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit”a