I'm pretty sure you can't use purify while you're imped by WHM (or charmed by DNC or horrored or whatevs it's called by RPR).

But yeah, the resilience buff after purify definitely ain't working right. It's got a delay for activating and you can get re-cc'd during that delay. My question is:
Why is purify even available as a button while you're not CC'd if the only theoretical reason you'd ever do that (resilience) is also tied to using it while CC'd

They'd either have to make resilience always pop up when you press the purify for there being a reason to even press it while not CC'd or they could make purify only available while you're CC'd. Either way would allow to make resilience kick up when you press the button instead of making it check for a condition before applying because that delay needs to go.