First of all: 
I am very happy to find out that the issues with the maps has been sorted out and that the P3Dv4 should work now (haven't tested yet).

Problem:  could not move/delete the setup executable nor uninstall the newly installed version 3.1.4 executable

I have installed (not as administrator) 3.1.4.129 in a separate directory C:\Program Files (x86)\Plan-G v3.1.4 as per given instructions.
It seems to be working OK sofar:  could load a flightplan, modify/save/export it etc.

But when I tried to move the directory in which I have unpacked the zip file containing setup_Plan-G_3.1.4.exe and the ReadMe.txt the problem started:  I was unable to move the directory ... no access rights to move of the setup executable.
Then I uninstalled 3.1.4 , but apparently it could not remove the Plan-G executable itself as wel as the uninstaller.  Same issue: no access rights.
The only way was to reboot my PC in diagnostic mode, then delete the files and reboot again in normal mode.

Did the installation again but now as administrator.  Now I could move the folder in which I unpacked the Zip file.  When I uninstalled version 3.1.4 the uninstaller did not remove the C:\Program Files (x86)\Plan-G v3.1.4 directory with the uninstaller in it.  Had to (and now I was able to) remove the uninstaller and then the directory itself.

Hope this helps.