It would be the very first original post's subject.

For example at the moment if somebody starts a thread 'man united team talk' and somebody changes the subject for one post to 'chelsea are great' when you view this in latest posts there is no way of knowing it was originally a man united thread.

This problem is more dangerous if the original thread was like 'naked chicks' and somebody changes the subject to 'look at this safe for work'.

Your case 1 and 2 went straight over my head I'm not too technical. Maybe the team will add it in a future update