This is kmail 4.14.11. The option "Mark selected message as read after __ seconds" would be better placed as an account property as I have accounts where I don't like to mark as read as some other person is the main owner of that account and needs to be informed about unread mails. This is possible for instance with the K-9 mail client.
Nope it will make kmail more complex. So I will not implement it. Sorry