Ironically, I think that Orodes II would be more likely to have Surena executed if he won a resounding victory (like he did at Carhae). A victorious general posed more of a threat to the regime of Orodes than a defeated one. A defeated general could just be exiled, cashiered, or otherwise disgraced.
The Parthians had their roots as a nomadic society. The Parthian King was not an absolute ruler like the Persian Padishah, he was the head of the seven great Parthian Clans. The Parthian King was more like the head of the Carolingian Empire than the old Persian Shahs.
By the way, great TL, keep it going. Just because Orodes II wouldn't be likely to execute a general, it certainly does not mean that he couldn't.