It is, because they have a non negotiable condition that A and B must be working in partnership, one as a therapist and one as group therapist.
I may have misinterpreted something in the post, I can give a crystal clear quick evaluation if I can be sure that I understand it correctly.
I understood this to involve a third original therapist involved as well, that therapist A was strongly encouraging disengagement from, although OP said that was not their intention when they brought A into the picture. If this is the case, then there really are several ways to approach this all of which the OP is clearly in the right.
If I misunderstood, and A is the original and B is the one wanting to be sole therapy provider for OP in the group, then that aspect isn't that relevant. Because A would have already put in text that she was in agreement with that agenda. It looked to me that A was the secondary and intended additional therapist though.
Either way, what I see from the posts is that OP did say they wanted to find solutions and was then left to figure out solutions alone, as neither therapist put effort into finding possible solutions. ( mistake 1 )
When OP texted that they couldnt see how they could attend group when the solution that they thought of alone was not acceptable to therapist B, OP came to quick conclusion the group would not work for them. Immediately upon relating that conclusion, which was not based on anything other than the inability to negotiate around a diagnosed disability, he was terminated abruptly by text for both therapist A and Group Therapy B. (mistake 2 )
The type of disability OP is an unusually easy one to accommodate as well, there were several different ways to handle going about that, as Social Workers, or at least B is, they know that. It's also the responsibility of the therapist to know who their clients are and what they would be appropriately placed in outside of therapy. She had 8 sessions to figure out what any disabilities might exist and whether OP could be accommodated in her group. If she pressured OP to leave original therapist without discovering that, then abruptly terminates when she realizes it doesnt fit in her and her partners agenda, its malpractice. ( and 3. now a lawyer likes you )
The abandonment law that you attached applies, it is disability discrimination under the law, you cannot leave someone with a disability to be responsible for disability accommodation themselves once a service has commenced and the provider had prior knowledge of it or reasonable time and opportunity to discover the disability before agreements were made for services, as they were here.
The fact that she said something like " I hear you are saying that you wont be able to attend " just before the abrupt dumping of her client pisses me off.
There was zero thought into jumping on an opportunity to passive aggressively put the onus on OP for opting out, thinking the wording in OP text legally allowed her to do that.
From what I see, and its just from what I see here, this is something that could be pursued. I'd separate the disability and go to services just for that issue. Then take the rest to malpractice, with the disability issue included. It's good to have that one on two burners at once. But, that is just me. I'm certainly not telling anyone to do anything. I can only validate the therapist is a b***c.