Pamela,
Speaking from professional experience, "serious emotional disorder" is what is in the Federal law. From personal experiences, it was only through the label SED that my ex-difficult child could get the help he needed for depression and many of the other things you mention (except ADHD.) The thing to watch out for is what types of students are in the proposed placement.
Roughly, there are internalizers (depressed, low self-esteem, negative, etc) and externalizers(challenging, impulsive, hyper, acting-out, aggressive etc.) and these two types of students do not mix well. First, the latter bully the former and second, internalizers need emotionally based therapy and perhaps cognitive restructuring. Externalizers need structure and positive behavioral interventions.
The problem with this nice dichotomy is some emotionally disturbed kids act out and some externalizers have underlying emotional problems. However, NOT having OSFA classes is still important.
When my ex-difficult child was young, I kept him in regular classes with in-school therapy (in addition to a lot of private therapy) because the BD class (no SED class existed) had all the bullies he was trying to avoid. Also, he had no academic problems that were not motivational in nature so he did not need intensive direct instruction of a special class (which is one of the good things that Special Education CAN provide--but doesn't always of course.)When ex-difficult child could not remain in public school because he was too ill, I made SURE that the EGBS I chose had an appropriate program AND students who were depressed internalizers just like him. No acting out behavior or aggression toward peers was tolerated. This safe environment was what these particular kids needed to open up and utilize the 18 hours of therapy that they had weekly. This program would not be good for every kid but it was very good for an immature, depressed kid with low self-esteem.
So if you think the new diagnosis is closer to the mark, then you should get an IEP FIRST with goals that address you child's specific needs (including in school therapy) and only THEN look for a place to deliver these services. Your SD should not be using diagnosis alone to dictate placement--which is what they are doing if they conclude your difficult child should go to a certain placement because of a diagnosis. It works the other way around: if difficult child has needs on the IEP that can only be met in a therapeutic school, then he should go there. However, some SED kids have needs that can be met in the regular class with support and related services (therapy and social work.)
I do not fear this diagnosis because therapy can work and kids can and do improve with interventions.
Martie