As updated in my last post, I had to create a fresh USB installer from the B10 full download (
http://swcdn.apple.com/content/down...ff81a690twrhri7k2vlmpkti/InstallAssistant.pkg), not the earlier delta update, then patched it with Barry K. Nathan's (0.4.1) micropatcher and then installed it over the top of the OS you're getting the -69863 error message on. Once done, boot into Recovery, mount your BS System volume with -uw / and use the 'diskutil apfs deleteSnapshot xxxxxxxetc --uuid xxxxxxetc' as you've tried before. Hopefully it will work this time.