Mairon/Sauron goes through a major change in canon, so it's understandable that he would behave differently at this timeline. Since his appearance varies in canon, a different appearance alone is making it difficult to call him AU.

The "AU-ness" is measured up against canon Mairon/Sauron, and not just the one occupying the canon spot, and our age down/up policy involves only physically appearing children or elders.

Is Mairon an earth maia in canon? Would changing him to a fire maia make him an AU?

I'm going to to send you a private plurk where we can discuss in more detail, hang in there <3