Hi Petertha,
I've no experience at all with air hardening steels so afraid I can't answer your question.
I can say that over the time I've been modelling I've heat treated (at home) using GFS (01) and Silver steel without too much problem overall though there has been the odd failure. I have a part not dissimilar to your own that I am concerned about distortion - the gear ring in the Bentley - but it has yet to be H/T. Made from GFS I intend to have that between two plates as described. If you go about it as suggested the distortion will be very small if any but the potential is there.
I made the thrust race rings from EN40b and had them Nitrate Hardened as this is virtually distortion free but then I was at work
EN40b is not a very nice material to machine to a good finish so wouldn't suggest that option.
A while back I was cleaning down a couple of diesel engines after their run at Forncett. One of them had run backwards and that action had unscrewed the rear rotor pin. Checking other the other engines pins I removed one and it broke apart in my fingers. Made from Silver Steel these pins had been heat treated with real care because of their function but I would guess the undercut for the screw thread reached a higher temp due to the smaller diameter and became crystallised. All engines are due for a replacement pins made from EN24t which is a very tough but machinable high tensile steel in it's supplied state. Whether that would be an option for a cam ring I have no idea - perhaps Mike could comment on that.
Regards - Tug