So any task with `Parent-Id` will be a sub-task. If User owns the tasks he can see the subtasks tree also. but if you own the sub task alone, for you it is like a normal task with an out of scope parent.

Recursive tasks works exactly like recursive calendar events.

I have not thought about Sub-Projects, any suggestions?

User-Group - That is interesting, I think it should not change the behaviour based on the Type of email, which is difficult to determine.

When a user leaves do the same thing as email - either purge them all or re assign it to someone else.