Glad you are better.
I would change the location away from Base 8 as it (in my mind) is more of a warehouse.
Shape changing based on emotions has been done before, but I still think the variety is original. Do the transformations give extra immunity? If not why is a head shot or teargas not enough to bring Markus down?
seen as a scapegoat for the devil
This might need rephrasing to
seen as a scapegoat or a devil
If I understand the practice of scapegoating you give all your sins to it and than ether kill it or drive it of. Both do not seem to match his parents behavior of keeping him.
If he was born with the condition he would be most likely be seen as cursed or as a curse upon the family, maybe even a divine test (Hiob), but that would not lead to the physical abuse you are going for. If his parents were not isolationists and had not hidden the pregnancy, there church community would have noticed the child going missing after birth. Also the role of his midwife is unclear. I am not 100% sure how the christian community you envision runs, but usually even there pregnant woman have a professional with them for birth and if they were not especially anti technology, they would most likely have been in a hospital.
There is a way to sidestep this by changing the beginning of his condition to after the baptism, with the family faking the sudden death of there child. Not very christian of them, but what do I know about their religious practice?
The bastard's broken free
Calling her son a bastard as a christian implies she is an unfaithful spouse. In German we call that an "Eigentor" (your words reflects negatively on yourself, beyond you insulting others).
Calling the police implies that officials ware complicit in the capture and holding of a child. If the police is not part of a GOI or a strange church, calling the cops in this situation is another "Eigentor" (indirectly admitting to the police that you abuse/isolate your child).
Beth called the police and died? Did Markus in his giant form sneak back into their house after he got out?
Also, in 18 years he never got this out of control before? No one came to visit them?
No "Sunday when the family is in church" breakout attempts? Depending on when he figured out to use his emotions to change willingly his parents would most likely not keep him from digging through a wall. 10 year olds are smarter than most think and teenagers are hormonal and difficult to control even if they do not have superpowers.
It might be that others will not pick at these threads, so no worries. My way of reading things is just a bit special if you recall.