I don’t think it’d really work to have a vision and then Mustafar, probably too chaotic. If one was to put the Vader’s helmet scene first I’d think a voice or voices would be more appropriate.
Yeah, I think you’re right. That way the “I’ve been every voice inside your head” line pays off.
In the theatrical version it felt a bit weird the first time, since we had never actually heard Vader’s voice coming from the helmet in TFA.