Okay, I got a response privately from a support person that I'd like to include here.
I still don't know when a release of AMDCompress will be made public that supports VS 2015. So I'd still like an answer on that.
Support response follows:
The issue with the DDS being 128 byte is due to the uncompressed source DDS file not being formatted as HDR as is the case for 2b_boat.dds (Format is compressed BC7) when converting to a compressed BC6H DDS file.
The SDK currently does not support direct Compressed format BC7 to Compressed format BC6H, It also does not support ARGB_8888 (uncompressed) to BC6H, This feature can be added in future releases.
2a_boat.dds is Uncompressed 32bpp BGRA
2b_boat.dds is Compressed BC7
2c_boat.dds is Uncompressed 32bpp BGRA
Please try using
EXR to compressed DDS (BC6H)
AMDCompressCLI -fd BC6H .\images\Balls.exr .\results\3a_Balls.dds