Sounds a little late for a derivation from Viking drinking practices.

The only citation I am aware of your "enemies skull as drinking bowl" is from Herodotus about the Scythians. A connection bteween head and container/vessel of some kind is in Romance word for head from Vulgar Latin testa 'bowl' which replaced words derived from caput. English crazy comes from a potter's term crazed 'marked by little cracks (in the glaze)'.

Ceci n'est pas un seing.