Tragedy developed out of choral performances. Action is very limited and usually takes place off-stage and is reported by a messenger vel sim. This would explain how he could be left on stage when the two actors departed, yet one would play his voice. If the same actor played Hephaistos and Prometheus, perhaps the emphasis was on them both being craftsmen. And whichever actor did not play Prometheus then would have played the rest of the roles.
Myth is multivalent.
He is a tyrant.