Oh man I love long responses! hahaha

I think Shinra seems more surprised that he didn't think about it before hahah. About the stabbing incident, it's actually Shinra who saves Izaya. Izaya just takes on the blame and promises to make Nakura pay all his life for it.

I think it's more that he lies to himself that he is above having friends. At some point in Ten, he tells Celty that he's lied to people around him and himself but he wouldn't lie about business. (God, I love how well written he is.) I don't think he can fool himself very well though.

I think Izaya not considering Shizuo human also points to another thing. He is set apart from the rest of them. He wants to be accepted by the rest of the people, he loves them as he says it. But I don't think he quite understands Shizuo's hatred of him is the same kind of hatred as his. He genuinely thinks that Shizuo would never accept him, so that is the ultimate way to protect his feelings. Acting like Shizuo is one of those monsters that he doesn't have any interest in. I think he can't convince himself in this regard as well, seeing as how he keeps muttering about it but can never actually get Shizuo to show it. Like you said they don't try very hard to murder each other. It's like they give it their all, but they still know that the other will get out of it. At some point, Namie makes an observation to herself. She says to herself "if you can't kill him with the knife, kill him with the poison." It's a good observation on her part that should tell the viewer that there is something missing in Izaya's efforts to kill Shizuo.

Yet I don't doubt that they hate each other. They hate each other because they are too proud to love each other. It's such an instant passion that Shizuo starts hating him when their eyes first lock from quite a distance (Shizuo at the school gates, Izaya up on somethingth floor through a window.)