Spoiler
The priest near the end can be examined. If you do, Tilana and Varsena ask who he was and Maestro explains he is Galarand's high priest, Isk Var.
The woman referred to doesn't really keep the monsters at bay. Neither does Galarand. Galarand's spirit is little more than a puppet or middle man for a greater force (the unnamed female). The implication is that this is not her only focus, but for unknown reasons, she's decided to become active in this area of the game world - channeling her commands through Galarand's spirit.
If you check Maestro's equipment, he has a Mark of Galarand. None of the monsters are literally trapped in the cave - when Maestro says he's free, he means from the compulsion to follow Galarand's orders. There is no longer a force capable of commanding him against his wishes.
He was commanded to do a number of things to weaken the forces of the land, but he found ways to manipulate people within those confines. With Galarand gone, Maestro is peacing out before anyone can shackle him again - and Varsena remarks that the force controlling Galarand may come herself now that he's gone. That's why she goes back to Elblum, and why Tilana tries to warn Aylan before returning to the monsters.
That very last part is implied, but I'm commissioning art to make it more clear.
Suven is wrong because he refuses to accept that Galarand is not the pinnacle of morality and righteousness he believes. By siding with him, he's ultimately falling to the same corruption Galarand's spirit has. He is not doing what he's doing because it's right - he's doing it out of bloodthirsty, vicious vengeance. He is no better than Marion blindly trying to kill Tilana without any thought to alternative solutions out of a sense of vengeance for her acts.
To Tanarex's point, I don't think Star Wars is a good parallel. Suven is not "going to the dark side" out of conflict with his feelings on family, though he does try to strike his enemies down in vengeance.
A better way to look at it would be to imagine that you are part of a major religion. You have an icon in that religion - a legend who represents all the ideals that you strive to hold yourself to. You've modeled your life after following their teachings.
Then you are told by someone who appears to represent everything your religion stands against that this major holy figure is actually a corrupt, tyrranical murderer. This person is even responsible for a great deal of hardship in your life.
So despite the fact that Maestro isn't lying, Suven refuses to see any grey area. No matter how corrupt or wrong Galarand may be now, in Suven's eyes, he is infallible. As far as he is concerned, he is still fighting on the side of light and good - not because he was "offered cookies." He doesn't experience a shift to the "dark side."
There is no extra final boss. The "she" referred to is not part of this story. Why the monsters invaded is purposely left unknown - but clearly a large contingency of them didn't want it and stepped up to fight against it.
Varsena isn't really a hero because she is willing to kill citizens of Elblum to make them fall in line with the idea that isolationism will lead to utter destruction. She has her beliefs where Elblum is concerned and by the end of the story, is willing to enact an "ends justify the means" attitude.
Maestro is purely acting out of self interest. He is the most selfish character in the story, consequences be damned if it benefits him. He is just good at seeing when helping others happens to help him.
Tilana is a tragic figure. She might be the closest thing to a hero, but even she is mostly just interested in herself. There's some hinting at the end that she will step into a leadership role for the monsters, perhaps to find a new, safe home for them - but she is also prone to vicious outbursts, and could use them for less morally right purposes in a fit of rage.
Regarding bosses...
Spoiler
Suven actually puts cure and cleanse on a 3 turn cooldown when he uses either. I might bump it up a turn or two, but I don't want Suven to too infrequently heal - he's been your major healer all game. I want him to act like it (he has a ton of conditional logic guiding his AI). I do think maintaining deadly wound or black flame to reduce healing is something I want to keep. I enjoy the gameplay of having to maintain certain buffs or debuffs while looking for openings to do more - a bit of a callback to skill "rotations" in MMOs.
I'm definitely still open to tweaking the last battle as I get more feedback - ESPECIALLY as I get LP/video feedback, as that gives me the best indication of whether a fight goes the way I intended.