I just tested a Linux Mint 18.1. I can confirm that there I get the correct
/etc/os-release. Also inkscape is now in PATH, and works fine.
On Linux Mint 18 it is also working fine, however I had to remove inkscape and reinstall, and restart the PC. I think it is important to mention explicitly in the documentation that after installing snap, either a restart or a logon/logoff is necessary.
I’m happy it worked. I hope Linux Mint and snap continue to get along.