Downgrading (user homes) to macOS Sierra
It will be very difficult to isolate the issue to individual preference files, since there could be several subtle issues.

Better to take the opposite approach - start with no preferences and only copy the old preferences over one by one as you come across them. That is, as you discover that you have a preference that needs updating, find the single preference file that will give you that preference, and restore it. Re-test and only proceed if things are still stable. Will be a drawn out process, but necessarily so I think. When you can no longer add individual preferences, consider the restoration finished and many set the remaining preferences.

This complex relationship between the OS, Applications and user files is why Apple created Migration Assistant. It might not be help here but it is useful to know just how intricate the process is of "migrating" from one OS version to another.