How to Launch Finder Process from Terminal

The following command will return the path to the frontmost Finder window, if that's what you're looking for:

osascript -e 'tell application "Finder" to get the POSIX path of (target of front window as alias)'

Note that this cannot return a POSIX path if the frontmost window has no POSIX path. So if the frontmost Finder window has an ongoing/finished search, is a smart folder or other magical location (like your Mac under devices, which lists all the mounted volumes and the network).

Search/Smart folder/No Finder window returns:

execution error: Can’t make «class fvtg» of window 1 of application "Finder" into type alias. (-1700)

And trying to execute the command when viewing your Mac:

execution error: No result was returned from some part of this expression. (-2763)

The open command exits after it opens Finder. Even if it didn't, closing a shell would close background processes. You could try using disown:

/System/Library/CoreServices/Finder.app/Contents/MacOS/Finder & disown $!

Or launchctl submit:

launchctl submit -l my.finder /System/Library/CoreServices/Finder.app/Contents/MacOS/Finder

launchctl remove my.finder removes the job.

Have you looked at log messages or crash reports? If you have tried to load or unload /System/Library/LaunchAgents/com.apple.Finder.plist, try removing entries for Finder in /var/db/launchd.db/com.apple.launchd{,.peruser.501}/overrides.plist.