A volcano isn't an antagonist.
You can fight the effects of a volcano, but Mother Nature isn't something that can be defeated. Only contained. Kind of like trying to have the villain of a story be climate change. If you want an antagonist, then there needs to be a person/ force obstructing Melayne from accomplishing her goal. A person from the endangered world who doesn't believe her and champions Shoran. Or a mysteriously sentient lurimont who is pregnant with eggs so Melayne can't blast her out of the sky. Or a Shoran minion that has been forcibly changed...a sorcerer child who can travel between dimensions.
That said, I consider your manuscript to be working toward a first draft. There is an author that I listened to (at the writers conference) who co-writes with another author. To maintain consistency, they write the plot first and add the characters/ conversation later. Because they are plot-driven (and not character driven writers), their work is about sweeping events that affect the MC without a chance of letting them misbehave and derailing the plot.
NJC's strength is in happenings. Not people. So he doesn't write about people. He knows what is going on behind the scenes, but he is getting the plot down first. Maybe/ possibly after that he can/ will consider adding a villain.
An example I can think of that is similar to this story is Sandra Bullock in Gravity. She is out on a space mission and debris kills most of her crew. Space kills everyone else. She is alone...woman vs space. One could say that the enemy is an unstoppable force (space), but the MC's worst enemy is herself. She fights herself constantly throughout the story...her depression...her fear...and her despair. Maroon Sandra Bullock on the ocean? Switch characters and that is Tom Hanks talking to a volleyball.
NJC, What I believe K is saying, IMHO, is that you have no antagonist and you need to figure one out. If Melayne's biggest obstacle is herself, then she must fight herself while she runs around/reacts to individual events. That means going into Melayne's thoughts a LOT deeper than you have accomplished thus far. Sweeping landscapes are a forte of yours, instead of depth perception into another person's soul. It might be easier to show us what is happening with Shogran, so that we see Melayne's actions block a plot or plan of the bad guy. Or introduce a 'little bad' that Melayne can defeat as she saves the volcano/ unstable dimension.
Instead of a soap opera, where time dictates the flow and events just kind of happen as they happen, an antagonist organizes the plot and narrows the scope, making it possible to break a big chunk of material into definable littler chunks. It might change the order of events in your book, but it would give your story HFN and HFA moments.
Just thinking out loud. Where you are going (right now) is a rewrite where you delve into Melayne's motivations and fears, making her fight herself. That is the route that requires the fewest rewrites or adjustment to the plot.