His IMMORTALITY didn't heal him, that's where his regeneration comes from. Zamasu injured himself with the lightning, half his body remained scarred as a result, and his immortality didn't trigger the regeneration.The moment Zamas uses light of justice, the narration states it didn't heal him TWICE [Vegeta/Gowasu and Shin] but is not regenerating. You keep stating the opposite that it powered him up with no text to prove your point. You may be right, however, TOEI's bad writing didn't state it. Therefore it is inconsistent and doesn't substantiate you. BTW, nothing mentioned about immortal Zamasu "overwriting" mortal Zamas soul when fused either.
And again, the whole of episode 67 debunks your theory. That's your problem. You take the statements of Gowasu and Shin at face value, when they were later on proved wrong. That's literally the point of episode 67. To show that their confidence was misplaced, and Shin and Gowasu pretty much give up hope in that episode, because they realized they were outplayed, that they didn't account for what would happen after Zamasu's regeneration was permanently shattered.
Anyway, let's agree to disagree. This is going nowhere and the others in this thread have moved on, so I don't see the point in continuing this.