I understand the option not to let an OP change the thread opening post because it can lead to people rewriting the whole thing and then it doesn't make sense with all the subsequent replies.
But I think it would be really useful for the OP to have the option to insert an UPDATE style post which would sit directly under the opening post - ie the first reply spot - but would be clearly labeled an update.
It could be used to quickly show an update or resolved issue. But more usefully it could be used to quickly explain to the type of poster who will not simply read through all the OP updates and is misinterpreting or not understanding the basics of the thread.
Current Mother's Day post is a prime example.
OP would be able to make it quite clear in her Update that the MIL was getting to see OP and her grandchildren later in the day, as well as clearing up all the other points which have been misconstrued over and over again.
Sure some people would still slip through the cracks but I think most would read an OP and a clearly marked OP UPDATE before replying.
Would make a lot of threads a lot saner.