Thank you all for your advice
. That makes a lot of sense; that redemption needs to be active before one is cursed. I have observed that the few monsters with redemption auras have substantially reduced curse duration.
So redemption doesn't work like cleansing, in that you can switch it on to reduce/remove curses then switch it off. That's fair enough, and it means I'll need to craft a holy orb for my sorc and hope to get relevant skill bonuses and mods (like fcr and +cold or specific skills I use).