As Tiko said, as it stands and for the very near future, not likely to change. However, it is not impossible for us to add the ability for staff to be able to view the edit history on reported conversation messages or conversations in which staff are included, naturally allowing us to grant for longer user ability to edit their conversation messages without it affecting the previously stated concerns.
However, this ability would be dependent on a host of other improvements to the site's search handling, which while being something I want to get eventually, cost a bit of money and aren't what I'd consider a glamorous cause for a donation campaign, so I've mostly keep it on the back-burner for when I can personally afford it.
So, TL;DR : Eventually edit time limit could be re-addressed to allow for what your idea requests, but it's a side-effect of other future improvements that are planned but no due date; Soon™.