@marcin should be back tomorrow, but might need some time to catch up on things... might be good to add another reviewer to this

we need separate functions for creating and storing olm sessions because we may not know the cookie ID at creation time

What is the example of such situation?

registered a new device on the keyserver and confirmed the notif olm session was stored in my mariadb instance

This should test that createAndStoreOlmSession works properly. How would you test that separate methods to create and store session work properly? Perhaps you should execute a scenario when session is needed before we have cookieID?