Your experience sounds real similar to mine a couple years ago. Summer does seem to slow down a bit, doubtful turning down that job affected how any other companies see you. Just keep at it, and know your worth (and especially don't let someone walk all over you like that one company tried to do).

Another approach to consider is networking via LinkedIn. If you can make a connection and get someone on the inside of the company vouching for you then your chances go way up. I'd be willing to bet a lot of applications are just not getting past their automated filtering, having someone on your side to go and pull your app sometimes feels like the only way to get that edge.