Unable to start after having applied all known recovery steps
Your Mac has Internet Recovery, so you can boot to that and reinstall to a USB drive or SD card while leaving your internal SSD intact.


Should that fail, you have a good case to seek hardware repair from Apple or someone else you trust.
Should that work, then you can decide whether to wipe the SSD and reinstall or perhaps use the clean OS to assist in cleaning up or backing up files in preparation for reinstalling on the internal SSD.


If you repeat your fsck in single user mode, it should eventually come up clean or with a permanent error. I would use the Recovery HD to run fsck / Disk Utility since that is less likely to have corruption than your OS (which still runs in a limited mode in single user mode). It's entirely possible you have software corruption that single user mode can't fix or a failing hard drive - but given the work you've done, I'd take a positive step by installing a clean OS on a clean drive before investigating the current OS more.