This is true. However, in his defence, I will say that he has no real idea of whether unicorns are sentient or not, and although it was remiss of him to assume they are not, under the assumption that they are not sentient it is a good plan.

Yes, though. It is out of character for Harry, who has in the past done things like become vegetarian when he though that there was the slightest possibility that animals could be sentient. He was still in shock, sure, but the Harry that we know should have known to ask.