Young!Lin's hair looks rather greenish to me. Not exactly Adult!Lin's shade, but it's not exactly black. And if I'm not mistaken, hair tends to become lighter or darker over time, so our favorite Steelix-killer wouldn't need hair-dying shenanigans.
So, if two Lins really exist, how would this work in-story? Something like "Young!Lin escaped from the orphanage, hid in the Grand Stairs and fell into a crevice. She eventually ran into the Citae Arc d'Asdfgh and met the same baby Arceus that Terra mentions in the Glitch City, becoming obsessed with reviving it and being split into two Lins, one that grew up as the Lin we know today and one that stayed behind and didn't age.", maybe? If I'm not mistaken, someone mentions at one point that young!Lin was a sociopath even before she joined Team Meteor, so this wouldn't be out-of-character. It'd be a bit weird, though.
If this turns out to be canon, Young!Lin should be named Linkastel.