I don't really know what got them to reply. Maybe it's a combination of Chinese, mutual respect and proved skills. Maybe it's just asking with a clear goal in mind. Either way, it got me something.
Last night I hurriedly removed some uC/OS code, but I don't think I correctly changed all the gba_screen references to UP_SCREEN. Also, I hurriedly added a ds2_main function which contained an audio init call of 256 samples at 44100 kHz. I don't think it will work correctly until I correct these values. I'll need to iteratively test this.
1) Yes. I'm using the code of NDSGBA 1.21 as a base, but heavily trimming it.
2) The Supercard DSTwo SDK is just a set of libraries, i.e. pre-written code you can call from your code. It is written in C, and so is NDSGBA 1.21.
3)

It will be released when at least a few improvements have been copied from CATSFC, no sooner. Else it's just NDSGBA 1.21.
Normmatt also has an idea for emulation speedup: "only the Div and DivARM bios methods are HLE [high-level emulation], enabling the others and implementing the other common bios methods in HLE should provide some speedup" - that remains to be seen