I'm not sure if deleting a message would also remove any children (ie. replies). I suspect it wouldn't and instead re-order the thread. It may be worth a simple test to find out.
As for what happens at the end of the edit window - using your Senate confirmation hearing scenario for example, that would be a case where the affected person could and should contact the forum admin for help.
The more I think about it, the better I like the idea of a reasonable window, but not left on indefinitely.