1) Yes, your application is the COM client, and Project is the COM server.
2) I'm not sure what you are asking here--what do you mean by "interop declarations"?
3) There isn't any default implementation of RetryRejectedCall.
4) No, you need to implement and register a message filter in your client. IMessageFilter is a strange interface because it has both client and server methods on it. You only care about implementing RetryRejectedCall. You will need to create a class that implements this (you can just throw NotImplementedExceptions for the other methods). Then you will need to P/Invoke the Win32 CoRegisterMessageFilter function to get it registered.
hope this will help you.