the more powerful spell is determined by its spellpower I think. Thus the increased crit rating from NMIC doesn't overwrite a spell that say was applied during a lightweave proc.
Yep, what they said. Throw a SoC, then another spell, then Corruption. You need a filler spell in between due to flight time on the Seed. It's a problem with the way Corruption is rolled. Basically every time it auto-refreshes it gets whatever current SP you have. However, it then gets stuck thinking you have those buffs up, so you can't over-write it even once all those procs are down. The Corruption still thinks it has say 4.5k SP on it, even if it's only ticking as if it has your current 3.5k SP. So if you try to over-write it won't let you, even though your SP is the same as the current SP on the DoT. Silly, but yeah. Seed over-writes no matter what, drop a filler spell to make sure the Seed lands, then Corruption. I like to do it on a Shadow Bolt since the cast time usually lets the Seed connect, then detonate so that you didn't spend SoC's cast time on 0 damage.