Hmm... maybe. That might be a good direction to go for Mystery Element, and to help straighten out Autumn a little, and maybe come up with more characteristics for Winter. But it still doesn't leave me with a solid idea of what properties to pick for those two. I like something-similar-to 'work' for Autumn to connect it to Iron through Strength; I'm very fond of the Fear link between Night and Autumn, so I would want Autumn to keep one property that supports that connection; and Winter needs a characteristic to connect it to Night through Sleep, plus one more to give Mystery Element something to hold onto.
Also, I want to fit 'air' somewhere, and I think it might work best as the link between Winter and Mystery Element opposing Iron.
edit: Further thoughts - The word "witch" is a little bit overloaded in Effulgence; not that I'm especially planning an Effulgence 'verse based on this fascinating little setting, but against the vague possibility that it might happen, it'd be good to have another word for People What Does This Magic Thing. Any suggestions?