Gotta say, my personal theory on Rennala's current state is that it's a consequence of the divorce itself. Miriel mentions consequences from breaking a Vow, all of Radagon's kids with Marika turned out deformed in some way. Vows and holding to one's word are deeply important in ER, as shown by many other enemies. It's why I think the Miracle of the Church of Vows could restore her.