@SamaraDraven; I read the rest of your post, and no surprise, I disagree. Hehe. These posts are getting longer, and I don't think either one of us going to change our minds in the end, so I'll just make this post as short as possible.
1. If anyone had jurisdiction over Anders, it was Aveline IMO. Not Hawke. Hawke doesn't even have a real office. Champion of Kirkwall is nice, but Aveline is the Guard-Captain. Dealing with criminals is her job, and she was right there. I don't think Hawke is more right than Sebastian or has any more authority than Sebastian regarding Anders.
2. I don't see Alistair as being any different from Sebastian in choosing to leave at the Landsmeet. He was one of only three Grey Wardens in the country, he knew that, at this point Loghain hasn't gone through the ritual, and still he leaves the Warden and Riordan to fight the Archdemon in the final showdown right around the corner. It was the same thing Sebastian did, and its understandable.
3. Sebastian wasn't just bumming around Kirkwall. He petitions the Viscount for support in Act 1. He doesn't find out until Act 2 that the Harrimans were behind his family's overthrow, and plans to gather info on the situation in Starkhaven before making a run at, or thinking about making a run at the throne if asked when he's returning to Starkhaven. The desire demon makes him doubt the purity of his decision and he has to sort that out. Elthina has been telling him to return to the Chantry in earnest, Sebastian wants to honor his commitment to his faith, has thought about all the reasons why he wouldn't want to be king, and since the desire demon episode, he's unsure about everything and is looking for guidance from the Maker on the issue. In Act 3, he refuses to leave Elthina behind to deal with the mages and templars because he's scared for her safety and wants to protect her. Once the mess was sorted out, he was going to leave because he's discovered/reasoned (whatever) that a king may serve the Maker better than a brother. I don't think this sequence of events makes Sebastian unreasonable. I think its pretty understandable and human.
4. It seems pretty straightforward to me that he's bringing an army to fight against what he thinks is going to be another Tevinter with Anders at the helm.
5. I don't think making a decision between being Chantry Brother and Prince of Starkhaven is an easy one to make, and he can't be both, so when you talk about leaders have more than one identity and role, its not the same because the point is that he can't be both. He can't be a Chantry Brother and Prince of Starkhaven at the same time, and whichever he chooses will define him for life. Its not a small decision, and he has to make it in a highly unusual pressure cooker circumstance. The other identity, one equally important to him, is gone forever, its not put on the backburner while he acts as king or gives a sermon. Its not a simple choice of occupation, its more than deciding what he wants to do, its deciding who he is going to be for the rest of his life. I don't condemn him for being unsure.
But good debate, and yay for Sebastian. 