Several of these questions have been answered in this interview with Shonda Rhimes (Warning, some spoilers!)http://ausiellofiles.ew.com/2010/05/...-finale-scoop/
"Let's shift gears to the actual crisis itself. I get that there's a certain suspension of disbelief that comes with stories like this. But I don't get why the SWAT team didn't take the gunman out after they shot him the first time. It looked like they easily could have gotten a second shot in there.
RHIMES: They didn't have a clear shot of him again.
But he was just laying there on the floor.
RHIMES: But Lexie was in between them. And then she got up and was still in between them. And she's running away and he's running away at the same time. So I don't see that they could have gotten a clear shot.
Fans are joking that Seattle has the worst SWAT team in the country.
RHIMES: That's a shame because we have these super awesome SWAT guys who talked us through everything. And we had a meeting in which I said, Seriously, it would be five guys in a group searching [the entire] hospital? That sounds crazy to me. And they said that's how it works. And for me, it made it so much more horrible because that hospital is enormous. And the idea that just five guys are going to save them is ridiculous on so many levels, and yet, that's how it works. When you're looking for a shooter, you don't have a bunch of people all spread out apparently."