Melanie is only under Victoria's control because of Marcus and Farm 218. If those threats are removed, then Victoria has no power at all over either Monica or Melanie.
So, in terms of Melanie getting free or not, it doesn't really matter that she's currently completely under Victoria's control. It only matters what happens in the future.
Yes, we have a limited number of endings, but we don't know exactly how many, unless I completely missed the announcement. And we don't know how many of these endings might be "good endings", as opposed to "bad endings". If there are other good endings, beyond just the revenge path, then those endings will also feature Monica getting free of Victoria, Marcus, and all the rest. And there's no reason at all why Melanie can't share in that final victory.
You're just making an unfounded assumption when you say that Monica could have a good ending, but Melanie couldn't.