The timeframe of the farm is truly absurd. There is a virtually zero percent chance that MT will be relevant when you get her to seven stars via farm. This is thematically no different than completely locking the character behind a paywall.
We could assume that at some point they'd relent and make her more available, but I think the message should be clear upfront -- this timescale doesn't work, it's not acceptable. Don't even propose it. Part of the critical line in a F2P is the line between giving the players something grindy but acceptable, and giving the players a grind that is fundamentally offensive.
Keep in mind she has to be 7* in order to push to G12, and it follows that therefore at some point in the future you'll need to push her to G13, or G14 for her to remain viable in the current content in the future. 26 months from now what will the gear and level cap be? And if you can't push her to 7* then she'll be too far behind to be useful (or potentially even useable at all).
