OK,
since it will take a while for Dan to look into and find a solution for dependencies of main and subtasks with recurring dependencies I applied a temporary solution for my shared todolist.
I removed the due date from the main tasks so it is more or less only a title for its subtasks. Also I locked the main/title tasks so nobody can check them and the subtasks with it.
So far so good. But I noticed that my title/main tasks don't stay locked after a while (They are not getting unlocked intentionally by other users. I hid this in their views so it can't happen unintentionally). Is the lock not permanently or gets reset when a list is reloaded, changed or newly opened etc.?
How can I make the lock stick?