Delete project in cascade

Yes and no.
Yes it is possible to delete in cascade : some cascading will be available for project managers, others will require extra habilitation (specific access / can force delete )
No it is not possible to delete project with real work : it is a constraitn that will never be avoided. You will never be able to delete any item with real work. If it is finished or cancelled, just set it as "closed" (idle)