Ooh, challenge mode with characters sounds neat. :)
I think removing a rank of Calm Mind would be fine. The idea of discarding a resource to get a bonus of some kind sounds like it’d be pretty useful. I played a round yesterday (early campaign) where I got the two hazards back-to-back after defeating only one Knowble and it made it essentially impossible to defeat anything else that run. Having another ability in the back pocket that might help with that scenario would definitely be good.