Regarding the aftermath of the opening scene....

Hmm, I'm not sure what you mean. When Tatum first runs into Sidney she says Casey AND Steve were killed. And in the fountain scene the characters also refer to them both.

Casey was more of the focus because Stu used to date her. They had history. Her boyfriend Steve was more anonymous- at least in terms of the plot.

Because Casey bore an uncanny resemblance to Drew Barrymore.

Probably because all the men wanted to bang Casey and all the women wanted to be Casey?

Steve-o wasn't played by a famous actor.