Basically as you conquer more and more territory, the reigning Shogunate becomes fearful of you. On a short campaign Realm Divide hits when you've conquered about 15 territories (Long and Domination is around 20). At this point, you get a slight hit to relations with every clan and this gets worse and worse each turn. Eventually you end up being at war with everyone. It's extremely hard/expensive to keep allies.
With this mod, Realm Divide has a bigger hit initially (-60 to relations) but goes away by 4 each turn until it eventually disappears. This makes Realm Divide easier to handle for the player as you can still maintain your allies, so it ends up being you and your allies v. everyone else, a civil war if you will.
As for if this mod is savegame compatible, if you haven't reached Realm Divide in that save yet, then it will work. If you have, then it won't.