We are a Coffee Circle customer at our startup and your only real differentiation from CC are the letterbox-friendly packages. I assume this means 350g or less? Nevertheless, good idea, but it will be expensive for you ;)
Communication between open and closed source processes runs via IPC. Since all the IPC functions are declared in the open source part, you can exactly check what data is exchanged between the processes -> You can see that keys are not transmitted.
You can still tell what exactly happens. Two separate processes might be used: one for crypto + network which is open source, the one manages synchronisation (which is the secret sauce of all cloud-sync services, if it works really good).