Hello!

You may use Jot Messenger without OneSignal, it is not mandatory option to make it work. We will add email notification to Jot Messenger in the next version, it will help to notify your members about new messages faster. 

About OneSignal: We are working on alternatives and try to find something in the nearest time. 

About Web Push and Mobile Push:  We are using only Web Push, because we have only web version and have no mobile apps.