I never said Dumbledore didn't know how to banish the lamia, but he was preoccupied with controlling hundreds of panicking students and was assured that Bagman could control the creature himself. No one realized Bagman's control was not as good as it should be until the lamia started to break out of the enclosure and Ludo couldn't contain it.  Severus reacted quickly to save lives, since he was the only other wizard capable of handling a dark creature that way besides Dumbledore. As for allowing dangerous creatures on the grounds, there were plenty of those in the books--the basilisk, the dementors, Aragog, and Harry faced them without any help from certain people, and the Headmaster was aware of all of them.

And where did you get the idea that Harry has bandoned Ron and Hermione? Simply because they're not always together in the story doesn't mean they're no longer friends.  But they both have other interests at the moment and aren't Harry's shadow.  Ron was a bit preoccupied with getting a date and Hermione with Draco while Harry was training with Phil.  But they ceertainly haven't abandoned each other, they are still good friends and were cheering him on from the stands. That part simply wasn't mentioned yet.