MacOS – Two Finder processes are open. Neither works. What gives

The finder.plist isn't in /Library, it's in
~/Library/Preferences/com.apple.finder.plist
That would be why you couldn't find it.

Quick way to get there…
From Finder
Cmd ⌘ N for New Window
Cmd ⌘ Shift ⇧ G for Go To…
Copy/paste ~/Library/Preferences/com.apple.finder.plist including the tilde ~
Hit Enter ⌅

I'd also consider clearing the Saved Sate at
~/Library/Saved Application State/com.apple.finder.savedState
It's safe to delete the entire folder contents - data, window etc.

I do not have FontLab Studio installed however for the purpose of providing a solution to your situation, under OS X El Capitan 10.11.5, I did the following:

• In a Terminal at /private/var/db/BootCaches, using sudo, mkdir and touch I created a directory named 34D6C504-2F5B-416F-9C91-0C9D019BF446 and in that directory a file named app.com.fontlab.studio5.playlist.

• In Finder, in my Home folder's Library > Saved Application State folder, I duplicated a folder and renamed the copy to: "com.fontlab.studio5.savedState"

In doing so this replicated the target locations and files with appropriate privileges to then test the following AppleScript application on.

In Script Editor I used the following code to create the Reset FontLab Studio.app and saved it in the Applications folder of the Macintosh HD and then dragged it to the Dock so as to have it readily available to click on.

try
    do shell script "rm -r \"$HOME/Library/Saved Application State/com.fontlab.studio5.savedState\" /private/var/db/BootCaches/*/app.com.fontlab.studio5.playlist" with administrator privileges
end try

When I click on the Reset FontLab Studio Dock Tile it showed the password dialog box, as shown below, for me to type in my password. I typed in my password and pressed enter. That's it, the app did what it was programmed to do.

Notes:

• As this code is tested I'd suggest copying and pasting it as is into Script Editor and then saving it as an application of whatever name you desire.

• $HOME is being used because ~ can/will at times error out in AppleScript and or Automator, depending on its specific use. Normally ~ is expanded to the value of Environment Variable of HOME and thus I typically use $HOME over ~ to avoid errors that it can/will generate at times.

• The substitution of * for 34D6C504-2F5B-416F-9C91-0C9D019BF446 is intensional, so as to accommodate the possible name change.

• The try command is used so as to avoid an error if you run the Reset FontLab Studio app again before the target files are recreated and or if one or the other doesn't exist, it still deletes the other.

• /var is a symbolic link to /private/var however I prefer using the absolute path over the symlink in cases such as this.

• The use of the rm command is inherently dangerous, especially when used with elevated privileges and or wildcards, so to that end make absolutely sure there are no typos in your command line and that the targets are the intended targets. This is why I tested my code before posting it. You can also preface the rm command with echo, e.g echo rm ... and then review the output in the Script Editor's Events/Results pane and then remove echo before saving the script.

The image below it to show syntactical highlighting and to see the full command line of the do shell script command

If you're the only user of the computer and you like not to have to type in your password when the Reset FontLab Studio app runs, then modify the do shell script command using the following syntax:

do shell script "command" user name "me" password "mypassword" with administrator privileges

Example:

do shell script "rm -r \"$HOME/Library/Saved Application State/com.fontlab.studio5.savedState\" /private/var/db/BootCaches/*/app.com.fontlab.studio5.playlist" user name "me" password "mypassword" with administrator privileges

Note that when the script's code is saved the value of user name and password are saved in readable text within the application bundle's "main.script" binary file. So use this method at your own risk as it is considered to be a security risk storing user names and passwords that are stored in an unencrypted manner.