I have to say this is actually a pretty cool spinoff idea. I'm really looking forward to seeing more of Mavis and her creation of Fairy Tail. While things are problematic in the current time of the series, the past was full of dangerous times. Get to see her create all those legendary spells and build things up.
Should be fun, well aside from all the death in the first chapter
. Would agree that it's possible this Zeira girl doesn't really exist. That kind of loneliness can be tough. Plus with the potential Mavis has, its entirely possible she created her via magic since we've seen that as being possible. Be it not stating her age right after Mavis or having Mavis state what could be an ironic line of "it's like I don't exist." Just feels like being isolated for 7 years would force Mavis to create someone to talk and spend time with.