We have stuck with mainstream for my son with mod-severe autism and delays in language, learning etc but have ABA provider who goes into school (provides programmes of work on autism stuff like social, language and behaviour + some of 1:1 support time). He's currently year 4 but doesn't go full time - he has 1:1 sessions out of school too.
There is no right school for my DS he falls between SS and mainstream (there are no units here). So what we have is a compromise and in many ways the least worst option. I've come to terms with the fact that the right school doesn't exist locally. Luckily for us the ABA programme compensates for many of the negatives.
In essence we have a dual placement of sorts with specialist teaching and mainstream setting. The benefits of mainstream are the other children, higher expectations academically and DS gets pushed out of his comfort zone more which helps him tolerate busy, noisy, unpredictable situations outside school. The downside is he spends lots of time doing bits of the national curriculum which aren't really a priority for him and sometimes are not functional. The local SS are for children with very severe needs so he doesn't fit there either. if he went there he would have a lot of downtime esp as staff ratio would be 4:10 not 1:1.
We moved schools at year 2 due to a poor attitude of head and teachers who just wanted ABA team and 1:1 to do everything and teachers to have zero involvement. New school has a HT with a child with Sen herself. That's made biggest difference and HT attitude trickles down to staff and children who are all very positive about DS being at school (even though it means more work, extra planning etc).
Local parent groups and forums can be a good way of finding out about schools and HT. I think you need to meet the head when you visit and get a feel for how they run things. Outside professionals sometimes give you a heads up about which schools are receptive to advice and which are not. you could consider dual placement or if in mainstream consider trying get extra 1:1 time eg for LSA to make resources, training etc on top of contact hours. For us ABA is ideal as the costs of outside provider are paid directly by LA so school gets a lot of extra support / specialist input but it doesn't affect their budget.
As our HT has a good reputation locally among Sen parents the school gets more than its share of children with extra needs. HT is also very good at getting funds out of LA so instead turning children away she makes LA agree to fund full time 1:1, breaks, resources time etc
I know what you mean about schools saying things to discourage you. We found this too. even old school was enthusiastic at first and then the emperors new clothes fell off and it was obvious that by year 2 they were interested in the other children's sats results and not bothered if DS achieved anything. I felt I had to prove DS was a child with potential who could learn. For a while I was effectively home ed and just sending him to school for social side. New school is much better on that front.