The problem is:

If I delete a WP (e.g. task), the relationship with a parent are maintained. In this manner I can't modify e.g. data in the parent.

Can you describe what you're seeing in more detail?

Are you able to see the task after deleting it? (I'm not sure whether you should be able to see it after deleting it or not -- I'm fairly new to OP myself.)

Great info.. Thanks for Sharing