APFS Errors: fsck can't repair

Same advice as the Apple forums. Backup and reformat your system.

Apple has not released an APFS specification or code yet. As you have found, only very rough reverse engineering has been done. Trying to interpret what key 2 miscompare in the fsroot tree is pure speculation at this point.

There is no way to convert back to HFS+. There is an unofficial way to opt out though.

There is no need for a fsck command means that the FS will try to fix anything wrong on-the-fly, online. No filesystem can be immune from all potential damage or implementation bugs. If something can't be fixed by fsck offline, then there really is no chance to have it fixed online.

However, one potential scenario is a bug with fsck and not the system code, but again that's pure speculation without being able to understand what's wrong. It's your choice, but a reformat is the safest point.

Try checking your RAM.

I had a similar issue with 10.13 (High Sierra) on a 2017 iMac (18,3) with a factory SSD and 3rd party RAM. I got repeated freezes, and Disk Utility First Aid reported errors like "Object map is invalid" and concludes "File system verify or repair failed". Each time, I would reformat the SSD and reinstall macOS and software, but the problem would return after a few days. It passed Apple Diagnostics, and my local Apple Store was unable to detect any hardware problems (after I removed the 3rd party RAM). I have had no problems on multiple other computers, including 2 other iMacs, a MacBook and several macOS virtual machines.

What fixed it for me was to match RAM by bank, i.e. Bank 0 (DIMM 0/1) contains a matched pair of Apple RAM, and Bank 1 (DIMM 0/1) contains a matched pair of 3rd party RAM.

Update 2018-01-06: Just got another failure after 1 week. (This is the trouble with random failures). I have temporarily removed the 3rd party RAM while I do more testing.

Update 2018-01-22: After 10 days with new RAM, the problem has not occurred. I believe my problem was caused by bad RAM. (Maybe APFS uses RAM as a cache?).