On the helmet - Face of a fool, potentially belted on, wearing a bunch of repeated medals. I'm thinking Dom might just be inspired by Loki. Instead of having his mouth sewn shut he had a metal helmet sealed over his head.
On the sword - So it looks like the longsword is grown directly from the ember while the greatsword is upgraded along the path the player uses. The crystal knight being of Zena would be an nice connection between Seath and Dom. I wouldn't put them as allies but they obviously work closely.
Also very disappointingly last night I had an epiphany about ... someone. Unfortunately it was on a way to a party where I got drunk and now have no idea what my epiphany was. Hopefully something will jog my memory.