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.