I wouldn't be entirely opposed, but I'm not sure it's necessary. Corinne, at least, is implied to have a measure of understanding of what Hazel 'says'. I was mostly leaving it up to everyone else individually to determine whether or not they knew sign language. Might instill some convenience, but Hazel wouldn't exactly be keen on someone that can poke around in her head.

As for Promethion, the Shrike's cannons are intended to be over-the-shoulder. Some boosters on the back, cannons over the shoulder, and the rest goes to making the forearms.