I can only use "C", the default, as the key for crouch. If I map it to something else, the key doesn't always respond. Sometimes I have to press it twice to crouch, but sometimes I don't have to. This happens to either crouch hold or crouch toggle. I much prefer to use Left Ctrl to toggle crouch like I do in many games. At first I thought my keyboard (Logitech G910) was broken in some way, but the same thing happened when I mapped crouch to one of my mouse buttons ("Mouse 4", a side button on Logitech G502 Hero).