As for Orsino's stupidity, I think that could be chalked up more to desperation than anything. Remember that they hadn't won already. On the contrary, there was still an army of templars out there and, from Orsino's viewpoint, the only thing he could look up for in the future was continued battles against even more templars, possibly an "Exalted March". At this point, he was probably suffering from levels of frustration and grief comparable to the ones that led the Arishok to finally attack Kirkwall. Also, he wasn't the "nice guy" he painted himself as, anyway. He wouldn't even know how to turn himself into some Harvester if he hadn't protected and cooperated with Quentin, the serial killer.
The difference between the Arishok and Orsino is that the Arishok sacrificed nothing to do what he did, and actually held steadfast to his beliefs and what his people and religion stood for.
Orsino sacrificed EVERYTHING that he spent basically his entire life preaching, teaching and trying to convey to the world at large. He undid it all.
On top of that, the Arishok was still the Arishok, while Orsino would forever be an abomination, win or lose. He not only gave up and sacrificed the lives of every mage in Kirkwall, but he effectively committed suicide in the process.
This is why the second act was far superior to the third, and end game act.
Also, there's worse stupid in this game, anyway. Sister/Mother Petrice and her buddy Ser Varnell are probably better contenders for the title of most stupid in the whole game, but I'd say that Thrask and Grace take the cake. After spending years doing the impossible, making templars and mages work together and all, they threw it all away by unecessarily hostilizing the Champion. First by taking a hostage, and then by atacking Hawke on sight. This being Act 3, after Hawke has already established a well-deserved reputation for being someone you DON'T want to mess with. Seriously, I can't imagine HOW they could have thought that plan could have worked at all.
In the case of Thrask and Grace, neither of them knew who was being kidnapped and if you look at it though, it is the Mages in the instance of the kidnapping quest that attack the Champion and do not allow for a peaceful solution each time the option come up. (It's almost as if the writers have something against wizards.
Petrice feels that she is protected by her status and the head cleric chick. I actually liked her involvement as it shows religious people as they are.
"I am devout to my God who is real, therefore I am righteous and justified, while these other people are heathens that are devout to their god that doesn't really exist."
It may be a dumb point of view and course of action, but it is a very real one.