The only thing I can think of Re. AC supply for VMID is some kind of frequency synthesis i.e not hifi use. I wouldn't worry about it. If I were looking to mod one of these, I'd use a filtered Vref and an op-amp buffer.

Re. the isolator, it is claimed to support "full speed" USB 2.0. Is there a difference between "full speed" USB and "hi-speed"?