From what I can remember, this book was about a girl who was set to marry someone but falls in love with another man named Viggo (I think) that lived up the mountain. And he was previously married but his wife died, anyways she stays with the dude she's meant to marry because she's trying to save the king or something and then in the end that guy ends up killing the king because they're experimenting on men that apparently failed a test earlier but no one knows yet they just think they're across this big toxic river? And then she and Viggo go out into the wilderness and cross the river because she's wanted and there's like super dangerous insects and they just go through it and then come across the lab with the boys in it and there's a whole lot of them, and they try to help them escape? And then the new queen or something is on a mad power trip.
Perhaps The Gender Game by Bella Forest.
A toxic river divides nineteen-year-old Violet Bates's world by gender.
Women rule the East. Men rule the West.
Welcome to the lands of Matrus and Patrus.
Ever since the disappearance of her beloved younger brother, Violet's life has been consumed by an anger she struggles to control. Already a prisoner to her own nation, now she has been sentenced to death for her crimes.
But one decision could save her life.
To enter the kingdom of Patrus, where men rule and women submit.
Everything about the patriarchy is dangerous for a rebellious girl like Violet. She cannot break the rules if she wishes to stay alive.
But abiding by rules has never been Violet's strong suit.
When she's thrust into more danger than she could have ever predicted, Violet is forced to sacrifice many things in the forbidden kingdom ... including forbidden love.
In a world divided by gender, only the strongest survive...
From this review:
Violet is forced to choose between death for her crimes or to go under cover into Patrus to steal back an item they stole from Matrus. Of course she will take the mission. Her contact in Patrus is Lee, whom she must marry for the plan to work. Lee is busy with the details of the plan and appoints a guardian that can look after Violet while Lee is at work. His name is Viggo. Enter the strong, silent type with the painful past.
I ran into a copy of the book online (not going to link it as it's an illegal one) and apparently Viggo's wife killed some nobleman who attempted to assault her, and she was sentenced to death. After her death, he left to city to live in a cabin in the mountains, which matches. And yes, there's something about boys in a secret laboratory, although it seems that the actual target of the machinations is a metal egg with an engineered embryo inside.
Found with a search for
site:goodreads.com mountain "viggo" marry toxic river