As I said this is not so much a real theory as it's a scenario I would like to see based on all the information we have so far, which isn't that much. There are numerous explanations as why she knows the name, she might even have given it herself.I would like this theory to be the actual case, But it doesn't explain why she knows Erza by name, unless She and Her lover"whoever he is" had it decided way before her birth akin to Roger and Rouge. I can't explain why but It definitely didn't seem like she learned her name recently.