It's been a couple of weeks of silence since I last heard from OSTA or the transportation company. I had sent them letters outlining the situation and my position, to keep for their records, but so far no response. Which is fine. My goal in sending the letters was to make sure they couldn't claim to have been unaware of the situation.
The original afternoon driver is still picking up special needs kids at Alex's school (and presumably other schools) which leaves me with a bad taste in my mouth. Due to privacy regulations, there's no way I can warn other parents to watch out for what she does with their children. The company is apparently fine with her actions (or running out her contract). But that is their decision to make and I know that this is not a fight I want to get into on a long term basis.
Alex's new afternoon driver is wonderful. She's cheerful and Alex likes her. There haven't been any problems with her.
It's good to know that my situation is settled, but I have a sense of civic duty which prevents me from feeling entirely good knowing that I've effectively passed off a "lemon" to someone else.