I think the shark blood thing was totally in character with Street's need to find a way to escape his circumstances. He already lost his chance to be on the wheelchair team (in a way, to go back to his old life, psychologically), he was no longer with lyla (so his connection emotionally was gone). The last thing he had left to try was to physically get his life back and since we all know how impossible that really is, the show presented it as such. But Jason, desperate for that feeling of fitting in, of being a leader, was at his breaking point, willing to do anything. I know when you say SHARK BLOOD, it sounds silly, but to Jason, he was willing to do anything at that point to get his life back, regardless of how unreal his odds were.
The nanny thing was an example of Matt choosing someone who cared about him and knew who he was over someone who knew Matt Saracen the hero football player. Was it a bit underdeveloped? Yea. Did I really think his other girlfriend was all that bad? not at all, the car thing was a real nice intro to here. Also, without it, we don't have the incredible EVERYBODY LEAVES ME scene.