I'm looking forward to Bastien's route! I know that him not being a noble is going to be a struggle for him but I hope he can still have the MC's back and defend her and stand up to others.
(Spoilers ahead) Yeah, Guillaume was bipolar, being sweet and a few scenes later he yells at her and expecting her to be his pretty wife to be at his beck and call (As if the MC doesn't have better things to do) God, and when he didn't put her name down to be under the Vilquine and was refused entry like WTF, how humiliating. I was looking forward for a scene in Leopold's route where she needs to come in the room and see that Leo more than likely did note her name down on the Stavizzi side (but alas she didn't have to enter the room.) Though Leo did invite her on his ally meetings which Guillaume didn't.
As for Guillaume getting better I did feel like it was a progressive, because MC had told him many times (or at least in my playthrough) that she didn't like him talking to her like that or pointed out that his Father was in the wrong. So when he had his breakdown, he spent some time being quiet and thinking about what he learned and about his mistakes. before talking to the MC and deciding to be King and apologizing for his behavior. The MC did forgive him quickly, I would have let him repent more and to beg for her forgiveness but well just agreeing to be King was a milestone for him and time was of the essence so it wasn't so. I wish he would have found out way sooner so he would have 'changed' sooner and they could have been more partners in crime like Leopold and the MC in Leo's route. Since I felt the MC had her hidden agenda (making Guillaume king) way longer than I would have liked since he took forever to decide to fight for the throne.
I also would have demanded to have my own throne, like wth, she did way more than him and she can't even sit in the damn chair.