[noggin]

Second, how is this little cheap device capable of properly playing back MVC 3D MKV when a far more powerful (and expensive) HTPC cannot? That doesn't make any sense to my brain!

I think there are two factors :

1. The Pi and Pi 2 have a GPU/VPU combo that supports MVC decoding in hardware. They are not unique in this, but without it it wouldn't be possible on such a low cost device.
2. There are a couple of developers working on the Pi / Pi 2 who have a very good grasp of the coding required to implement the hardware MVC decoding required and who are capable of coding for the GPU/VPU and the Pi firmware. (There are some Kodi/OpenElec developers who are very close to the Raspberry Pi foundation and/or Broadcom)

I think 2. is the key factor here, as other GPU/VPUs have MVC hardware decode functionality, but don't have Kodi/OE developers who are as close to the hardware. Also ffmpeg devs don't really see 3D as an issue it seems, so there isn't a push for software-only MVC decode development. HEVC is getting a lot more attention...

If you look at how quickly stuff is developed for the Pi, compared to Intel/AMD/nVidia graphics drivers, you can also see the benefit of 2.