I have noticed the same file/directory structure when I burn an AtracCD. As for bitrate, I'm not sure if there are "300kbps" bit-rate as HiSP Atrac3+ is only 256kbps and the original SP is 292kbps. OpenMG and SS can transfer the song as a "fake" SP, but only SS2.0 that support the HiSP. As for the player supporting higher bit-rate, I have an "older" gen atrac3+ PCDP, and at that time, atrac3+ only available as 64/48kbps. Recently I downloaded SS2.0, and my PCDP plays 256kbps atrac3+ just fine, which means your player should be able to play the higher bit-rate since it's still the same codec.
Although I don't have the NW-MS70D, I do have the NW-E3, an older Network Walkman, and I don't think you can upload songs to other PCs due to the check-in/out process.