....With that in mind, what is the max depth that training should occur/certify someone on with air?
I'd say anything deeper than 100' in training should require special focus on the effects of Narcosis on that diver, that day. In addition, the general ability of the diver should be taken into account. I've seen many a OW diver going for their AOW flopping around at 80' and headed for +100' depths.
Deep water is dangerous, no matter the training, skill level or gear configuration.
A divers training and experience mitigate the danger but none the less, the danger still lives down there just waiting for the hapless diver to make a mistake.