Ironically I would argue that if "the problem of generals" was the main reason for why the Twins treat Rowan the way they do, then they should honestly just be straight up nicer to a "good Rowan" that isn't actively trying to saboutage their major goals.
Think of it like this, no matter how high Rowan's utility might be, he should never be popular enough to incite a coup centered around his figure as long as the core of the Twins' corps remain "forces of chaos". If Rowan is trying to prevent looting and ravaging and pillaging at every turn then the orcs and demons will never get behind him against the Twins. If anything this would be the best case scenario for the Twins because they get to reap all the benefits of Rowan's sound administration while mocking Rowan's "orderly values" and placing the responsability on him for any decisions that are unpopular with the chaos races.
In this scenario, it makes perfect sense for the Twins to act antagonistic to Rowan in public, but behind the scenes they should definitely be trying to be more amiable to him and treating him well for his good job, instead of constantly harassing him for his values and trying to "break" him, because if they do that then they either fail and push him to a breaking point where he rebels against them, or they suceed and he becomes corrupted and then he has a real chance of becoming popular with the chaos forces and pose a threat to overthrow them, it is a lose-lose situation.
Maybe something to consider for chapter 2 anyway, a path where Rowan has demonstrated his loyalty to the Twins but isn't really keen on the whole "become an evil piece of shit and revel on the pain and suffering of others" that the Twins keep trying to force on him.