I didn't feel that the characters were stupid at any time of this arc. You must notice that they don't remember the past iterations. What they did in the current loop may be different enough to break the loop, but they can't know. This uncertainty makes Kyon think that doing nothing may be a good option. And I can't believe that doing homework was an obvious way to break the loop, even spending another day together with the brigade wasn't obvious since they have been doing this for 2 weeks. Our extern point of view makes us think that it was easy to guess but if you try to put yourself into kyon's shoes, things are really not the same.
Besides, Kyon was the only one who was supposed to do something. Yuki have to observe, Mikuru is lost because she can't receive orders anymore and Koizumi seems to be enjoying the loop.
So I don't understand people saying they are losing on purpose. Koizumi may be, but he would be the only one.
I even think this arc was good for the characters. I like Koizumi much more now that I've seen E8. And the characters showed some of their weaknesses in this arc, this is one of the reason why I liked it.
I think the amount of variation between loops was almost perfect. Enough to enjoy the difference, not too much to remain faithful to the novel. And too much variation would have killed the suspense since we know (among others) that the frog costume is present in the club room afterwards.
It was a really weak story in the novel, they made it awesome
PS : I registered to comment on this thread but I forgot half of what I wanted to say xD. Sorry if my english is bad, i'm not english-speaking.