1. If you take a regular user and grant him forum permission to approve posts, he can see softdeleted posts, can restore them and also can permanently delete them, despite the user doesn't have permission to delete or softdelete posts.
2. If you take a regular user and grant him forum permission to softdelete only (no approval and/or permanently delete permission), the user gets the X (delete) button in posts. But when the user clicks it, he gets a message that he can't delete posts in this forum.
It seems that permanent deletion of softdeleted posts is handled via posts disapproval feature.
Probably there're more weird behavior around that which I didn't test.
I think the behavior should be consistent with the user/group (forum) permissions system.