Yeah, that's a detail I'd have added at a higher level for sure.
As for the hand gestures-
GW have always used the heavy metal salute for their magic gestures.
But back then, they also features space marines with guitars, and there was even a Sabbat single as a free White Dwarf promo.
So it isn't like
GW's designers and sculptors don't know that it is a heavy metal salute- they're embracing it.