Can't open Preferences in Safari 11
I would be willing to bet that Safari 11 can't read the Safari 12 preferences file(s). No doubt part of the upgrade process is to make changes to these files to reflect the differences between the two versions.

Of course Apple is not going to tell you about this because going back to an earlier version who would do that? (GASP!)

You are probably have to play the preferences dance, eg start removing preferences files associated with Safari until it starts working again on your Mac.

Open a Finder window, hold down Option and go to the "Go" Menu and select Library. In that folder find Preferences and then look for things that start with "Com.Apple.Safari" on my Mac it looks like this.



It could be a little different on your Mac any one (or all) of these files could be unreadable by your version of Safari. So quit the app, move one of these to the desktop and relaunch Safari, until it works.

More bad news: there is also a "Safari" folder in Library, any one or more of these files could also be a problem. SO you may have to repeat that routine again with the files in there.