1. DXSI and TGSI aren't identical though, they are just close, the opcode set is really a union of DXSI and GL programs. Also DX9 is not DX11, their current design is DX9 based, they are internally developing something better, but again non-open development process. So writing an implementation of an interface that is fixed and I can't modify didn't seem like something that would be good value for my employer.
2. VMware haven't shown any interest in open sourcing any parts of their device beyond the Linux guest bits, and lots of code close to Mesa. I've talked to their engineers plenty of times and I've never seen any indication their management would let them open source their virtual GPU design process, or interact with others on it. I invited them to discuss open source virt GPU devices years ago, and they said they couldn't talk specifics.
3. Last I looked the VMware Windows drivers specifically say running their drivers on non-VMware implementations is against the license. That was one of the other reasons I started again, since if we have to write Windows drivers from scratch it may as well be for a device that can actually run DX11.