What to do when an inspector misses stuff?

I am sure each state is different, but it is difficult to successfully sue inspectors. Although the threat of lawsut might help him either file an insurance claim for you or help out with the repair.

As far as the agent - are you talking about YOUR agent. No reason to sue your own agent, and I am not sure on what basis you would sue the seller's agent.
Talk to your agent about the seller's disclosures. What, exactly, did they disclose about the discoloration and/or the roof? If they did not give what looks like a good faith disclosure then it would be worth talking to them. Its always best to use your agent to talk to them. Talking to them directly can hurt if a lawsuit is filed.