Well, I think Moash can't redeem himself, but there is something in RoW that got me thinking... when Navani sings that anti Odium rithm fixing the defenses of Urithiru, he loses his connection to Odium.
Perhaps if Moash is exposed to that rithm while in the middle of a fight or some sort of key moment in the fifth book, well, that could be a good moment for redemption. Maybe saving Gavinor or something like that.
Cheers!