I can`t say anything about the helmet, not my area but, I can tell you it would be best to just leave the liner as it is. Anything you put on that leather will have adverse effects at this point. Most leather treatments will darken the leather. Some treatments might make it softer and more pliable, but the drawback is the cells in the leather break down and leather will deteriorate faster. If the leather has dry rot, there is not much you can do, other than
keeping the helmet/liner in a controlled environment ( low humidity, no extreme heat) is idea. Others here may have more to say on this subject.