Peachy I know how hard it is when your child is like that, and I haven't even got any other little people around to protect. I can't even imagine how hard that is.
I wonder if the SS route to go down is child protection rather than disability iykwim. Here, they're two separate teams and child protection can also authorise respite in situations like yours.
I also think your SS disability department are breaking the law in refusing to assess ds1. ds1 is a child in need in official terms as he has a disability and they have to assess. I know having the energy to fight and fight is shit, but your MP is good, isn't he? Can you appeal to him for help?
A safespace might be a good thing for ds1, if you could get funding from anywhere, but I know it's hard without SS support.
I guess finally, the point I know I go on about: medication. It has helped J massively. He used to headbutt, bite, slap, punch etc. etc. every day. He used to want to stab me and go for the knives and I used to have to restrain him for ages, as did school. Medication has helped him to be calmer and he is himself, but happier. Maybe ask paed at your next visit.
I'm so sorry it's so shit.