This isn't true about needing both parents to change schools. It only takes ONE parent to register and deregister children from schools. Anyone telling you otherwise is wrong. Schools may want both parents to agree, which cannot always happen due to abusive relationships, but legally speaking, only one parent is required to register and deregister. I'd be seeking advice from a different solicitor, one who actually knows what they are talking about.
As for your ex-DP - he's just telling you what you want to hear, it's all classic script from these abusive men, I've heard it all before.
The £150/mth allowance is a joke, he's KEEPING all of your income from a business that you run. What was his reasoning as to why you cannot have your own income???? He's financially, sexually, verbally, emotionally abusive.
I know you're not going to the police, but you really should. You have the text messages to prove his threats. You need the police and social services on your side. By doing nothing, or worse still, by going back, you're just walking straight back into the lions den.
I come from a place of caring, I've been in your shoes, I've wanted to believe he'll never do it again, but believe me, men like this will ALWAYS do it again.
Please, for the love of god, DON'T GO BACK.
ETA Please don't get upset, but telling him your thoughts and plans was a mistake too. Do not give him any information regarding your thoughts or plans.