I don't understand why a previous prosecutor offered him a deal in the first place
I understood that it wasn't that they offered him a deal as such - they thought there wouldn't be enough evidence to prosecute so they told him they wouldn't and it would just be a civil case, then they later used his testimony from the civil case deposition (where he spoke more freely because he thought the consequences would only be taking a financial hit) when others came forward.
So I guess his lawyers' argument this time is that the testimony that put him away is inadmissible as it wasn't obtained fairly and/or could he said to be entrapment.
So the prosecutors were kind of right in the first place that there wasn't enough evidence to put him away.
It's really difficult when it's a "he said, she said" situation, and I don't know what the answer is, aside from building a society where women feel able to report sexual assaults immediately where there is more chance of obtaining evidence. This requires more awareness and a shift in people's values towards women and misogyny.