I will nitpick this specific statement because amount of investment is not tied to strenght. As Harmony who has two polar powers says he cannot direct his streght due to the will of the shards in opposition. As to the argument
I agree. Gavinor being odium champion seems unrealistic. Taravangian would not throw the fight unless he is aiming for something different. So if he uses Gavinor then he would have to do so to strike at a different target. If gavinor dies then Adolin replaces Gavinorn as heir. It doesn't help Odium.