He's supposed to be a patriot, so it's not a forgone conclusion that keeping him will lead do war. At least not anytime soon, the country would probably not withstand another one right now. Honestly, the only scenario in which that would happen is if he decided to stage a coup and install himself as king, which is unlikely to succeed as long as Aura keeps the public order and the status quo among nobility. I don't think she considers him to be that kind of threat, he hasn't even really aligned himself in opposition to her. Also, she'd be mad to give him any further honors and power if that was the case. A ruler has many ways to slowly marginalize a vassal over time and make it unprofitable to support him. It would only make sense to begin right away with him, if that was her concern. I think she's just worried that his ambition will lead to him being a big pain in the ass to manage and keep in check, therefore costing her resources and attention that would be better spent elsewhere. We've already seen an example of this - he basically trolled her with that mobilisation order, making her think that it will undermine the country's food supply. In the end the order was reasonable and beneficial for her as well, he just wanted to see her squirm.