There's a staggering amount of ignorance here about the challenges faced by SEN kids and their parents in trying to access education. In our case, it took a 6 week period of school avoidance, and the complete mental health breakdown of my then 6yo daughter before our struggles were taken seriously (undiagnosed, "fine at school"). Our (part time) senco told us not to bother applying for an echp needs assessment because they had no evidence to submit in support (they did, they were just focusing on the lack of evident learning disability because academically dd is bright). She told us that without a diagnosis it was fruitless. When she realised we weren't taking no for an answer, she asked me to write her referral form - I mean, I actually sat at her computer and typed up everything! We got the ehcp needs assessment, the Ed Psych report noted lots that the school hadn't seen, and we now have an ehcp.
Op, get that ehcp by hook or crook. If the LA turned it down, presumably the next stage is Tribunal? Ours was turned down initially, but I spent time going through their response and highlighting the litany of errors it contained, sent it back to Tribunal with additional evidence. The LA overturned their original decision before it went to Tribunal. I think you should focus your efforts on securing the ehcp first and foremost - using the involvement of children's services as additional evidence that her needs are not currently being met. Your daughter may even need a specialist setting, given that mainstream isn't working for anyone. The ehcp will be the key to unlocking more of the right support.
You are right, off rolling is a big issue - many SEN parent either choose or are forced to homeschool for this very reason. I don't even really blame the schools, it's the lack of awareness and resourcing at LA level that's to blame. Mainstream schools can't cope with children with significant additional needs, but there's a lack of alternative provision. Some are definitely much better at providing support than others. Our dd thrived in Y2 and has just moved up to Juniors from infants, where the support and understanding is truly excellent. I am fearful though of what the change to secondary may bring. But that's precisely why we fought so long and hard for an ehcp (she's also now on the waiting list for autism assessment).