Okay, I figure you have two options.
OPTION 1 - Delete Finder preferences
Let's delete Finder preferences first and see if that solves your problem. Since you can't launch the Finder, you will have to do this using the Terminal. Follow these instructions:
- Open Terminal (it's in the Applications > Utilities folder).
- Delete the primary Finder preferences file by typing this at the Terminal prompt exactly as I've typed it: rm ~/Library/Preferences/com.apple.finder.plist
- Press Return
- Since you're running El Capitan you will also need to delete the preferences for the Finder Sidebar by typing this at the Terminal prompt exactly as I've typed it: rm ~/Library/Preferences/com.apple.sidebarlists.plist
- Press Return
- Log out, then in again.
- Empty the Trash.
Hopefully this solves the problem. If not, proceed to Option 2.
OPTION 2 - Replace the Finder app
- Manually go to your Time Machine backup drive.
- Select a backup date you know was after you updated to El Capitan 10.11.6 and before you started having problems.
- Within your backup drive, navigate to the Finder app. This should be located at: Backups.backupdb > 'your computer' > 'backup date' > Macintosh HD > System > Library > CoreServices > Finder (Note: Your HD may be a different name to Macintosh HD).
- Double-check that the version of Finder is actually 10.11.4 (that's the version for El Capitan 10.11.6).
- Select and Copy the Finder app.
- Now, navigate to the CoreServices folder on your main HD. This should be located at: Macintosh HD > System > Library > CoreServices (Note: Your HD may be a different name to Macintosh HD).
- Paste the Finder app into this location.
Hopefully this solves the problem. If not, you could try Option 1 again now that you've replaced the Finder.
Let us know how you go.
Option 3 - Test Startup/Login items
Okay, I'm adding this option based on the fact that the Finder seems to operate normally when your computer starts in Safe Mode.
This tells me it's likely to be something that's getting loaded during normal startup (e.g. a Startup/Login item, a font you've installed) that's causing the problem.
So I would now startup normally and go to System Preferences > Users & Groups. Take a note of your Login Items and then remove all of them and restart your computer (Finder should still crash) so that it starts again without those login items loading. Test this and see if the problem persists.
If the problem does persist, I'd then remove any fonts you've installed yourself.
Let me know how you go.