We finally figured it out. It seems that, when I tried to steal the mage's power, I accidentally also took the rune's. It was July 3, so I managed to get rid of it that evening by putting on an early fireworks display as far out in the woods as we could get. I almost died of heatstroke before then, muffled in ten layers of clothing, which still didn't do much to hide the searchlight the rune had turned me into. I'll never be able to complain about Heidar glowing again.
Nonetheless, I was glad to have the rune gone. No matter who had finally ended up with it, it would have been nothing but trouble. Heidar plans to tell his dad that it was a fake, which is what most Fey seem to have believed from the start. I don't know what Tanet will say. He left shortly after our little adventure, having had his opinion of the dangers of the human world strongly reinforced. I don't think I'll be getting a brotherly visit anytime soon.
Heidar and I talked it over and decided to stay in the human world for awhile. My motivation is pretty simple: from what I understand, my twin is far more likely to manifest in Faerie than here. And although Heidar keeps telling me that I will come to love my other half, I'd just as soon avoid another journey of self-discovery right now. At least until she loses some of her baby fat.
Heidar's reasons for staying are less straightforward. He says he doesn't want me back in Faerie until he can take some precautions against the Svarestri. They don't know as much about the human world as other Fey, so he thinks we'll have an advantage, should any show up here. But I think he's really trying to work up the nerve to tell his father that he's going to have a daughter-in-law who occasionally goes scaly.
I'm still eating tofu, even though my twin is heartily sick of it. At least that's what I've been blaming all these new cravings on. I haven't resorted to grocery shopping at the pet store yet, but steak is starting to sound really good. Rare steak, with pickles.