Problems with partner caused by step child and his mother

I'm really sorry for the novel I just don't know exactly how much of the backstory is important and whether I need to vent or advice or support

I have 2 children (10 and 8) from a previous relationship which was extremely controlling and abusive. Their father is in prison for violent offences caused by drugs and alcohol and he hasn't spoken to my children in nearly 12 months. We separated 5 years ago.
My eldest is high spectrum autistic and mimics others behaviour to try to blend in.

My partner has 3 children (12, 11 and 9) from a previous relationship with a woman who has always been very manipulative and controlling. I know this because we all grew up together and she was well known for being a violent bully. He doesn't believe the younger 2 are his but refuses to get a DNA test as she will stop him from seeing the kids regardless of the results. They separated nearly 10 years ago.
She has caused as many issues as she can between us even trying to make him think I'm seeing other people or that I've logged into his Facebook and blocked her. She befriended my exes sister and tried to use me defending myself as way to say I'm violent and shouldn't be near the kids she's also remarked that my kids should be with their fathers extremely disturbing family.

My partner and I have been in a stable relationship for 3 years and have a 1 year old together.

2 years ago I agreed to move interstate so he could see his kids (she was refusing access because she didn't want to travel for an hour to drop them off). I've found it hard to fit in and make friends where we are as its a very small minded and religious community. I've never been married, have 3 children with 2 seperate fathers and don't attend church nor would I force my children to. I can only work part time as I am my sons carer and work in general is scarce here.

In the last 2 years we've had the kids try to push the boundaries to see what they can get away with and with persistence they've realised the rules aren't changing and if they don't want to be in trouble they need to follow them. They're great as a family now... Except for his middle child.

This child has continually tried to manipulate to get out of visiting us and done everything in his power to push me to my limit for the entire time we've lived here.
It started with little things that I thought were coming from the mother like insinuating I wasn't really pregnant and rallying the youngest to confuse my autistic child by saying we aren't a real family, they'll never be siblings and that the baby isn't their real sibling. It's never ceased only escalated and it's only ever when my partner isn't around.

The child has tormented my autistic child emotionally mentally and physically for a long period of time and when my child fights back he cries to his mum that my child has hit him.
When our baby was 5 months old I saw him kick toys in the babies face and when confronted about it lied straight to my face and put on the tears because I didn't believe him.
He told my children the Easter bunny and Santa aren't real, my kids asked me so I told them the truth and explained that now they knew the truth none of them would be receiving gifts from them. He went home and told his mother I told them they didn't exist so I didn't have to buy them anything.
When he get birthday and Christmas presents I'm the one planning, buying and wrapping them because that's how our finances work, not once has he ever thanked me.
On Valentine's Day my partner gave me flowers and because it was obvious we were happy he decided to steal money from my daughter, I knew it was him but couldn't prove it; when he realised I wasn't going to give in and he would be missing out on what the other children were enjoying the money was miraculously found in a place I'd already looked.
He has invented stories of being too scared at our house because we're apparently so violent and all we ever do is fight, this kid has even looked me in the eye and told me if he gets rid of me he gets rid of the rules and dad will buy him anything he wants. I've been told on a number of occasions he couldn't possibly do those things because he gets good grades at school, no one seems to realise being intelligent doesn't make you a nice person and the mother thinks he can do no wrong that I'm "just a bitch".

It has gotten to the point that I'm on a prescription medicine for anxiety because I'm crawling the walls trying to escape for up to 3 days before he arrives.
To top it off my autistic child has started to mimic this behaviour when at school and of course is getting in serious trouble for it and while my partner brushes this sort of thing off with his own kids my child is being severely punished. I'm in agreement about the punishment however I believe his kids need it just as much.

I want to go back to school so I can build a carer and future for my family and also myself but where we live doesn't provide the courses I need to get to where I want to be. I wanted to go back to where I consider to be home but that was too far away for my partner so we agreed at the end of this year to move to our nearest capital city.

Last night my partner and I got into a massive argument because his middle child told his mother he feels abandoned and doesn't want us to go so he wants to apply for a permanent position and I told him I refuse to stay.
He says he wants to compromise but doesn't seem to understand I've already compromised not only by agreeing to come here to a place I absolutely hate but also by agreeing to move 3 hours away rather than 10.

Right now I'm sitting in my car on the verge of tears, watching my older kids play at the park contemplating just going home and packing up; leaving everything we've worked so hard for and walking away from a person I love so fully, who is the only real father figure my children have ever had to look up to and never coming back.