How long you had the helmet? Any warranty? Think SDS might replace it?
IIRC the stunts are styrene plastic while hero is ABS? if so, be cautious about using products for exclusively ABS on styrene.
Styrene certainly does not have the UV resistance of ABS. Heat generated from Vac forming probably does nothing to help and could be a reason for variation in yellowing from one stunt to another. I've had some sheet styrenne yellow over a couple weeks while sitting on a work bench while other pieces in the pile have be unaffected.
Painting probably is your best bet. Might want to investigate that new Krylon spray primer for plastic. I've used it on outdoor furnture and PVC stuff with pleasing results.