Most sites that "delete" things only give the appearance of things being deleted. For various reasons including dealing with abuse, most things need to be retained.
Even if you could delete the things you've sent or received, the other person would still have it (one reason why it's typically a flag rather than being actually deleted).
But for those who like to have a clean mailbox I can see it being useful so will look at what I can do.
The post editing limits are also to prevent abuse and 'game playing'. I do hope to get away from the fixed limit so things can still be edited later (but the change made need review / approval to keep the protection)