I'm trying to figure out the possible ways to add functionality to InDesign.
So far I know about plugins. I know that there is no SDK for CC yet.
1. Can I develop a plugin for InDesign CC without an SDK or using an older version of an SDK?
2. Is there another way to add functionality (like a toolbar)?
Look at the Wave Tool project in 6 SDK. If your application works in 6 it should work in CC.
It would be a good idea to see if the functionality you are looking to add can be done through scripting. It can save an enormous amount of development time.