Just wanted to update you on this one.
PMs do not update last member post. So that one, if its happening, we would need to take a look at. However with regard the updating of member_last_post, this is not something that is removed if a post is then subsequently deleted. After all, the member did indeed post.
When an item of content is deleted, we don't demote members based on this. In the same way we dont delete post counts and never have based on the deleting of items. At present this is working as intended.
I realise this is not the outcome that you would have liked, and please feel free to post this within the suggestions forum if you would like to see this change at some point in the future.