I had to solve this exact problem a couple of months back. Multiple .mer files on a PVP7, no clear indication of which one was running. OEM did not provide a password to allow me to enter configuration mode to see which one was loaded, and all of my usual get-to-config-mode tricks like pressing the white square a bootup, or Ctrl+Alt+Shift+Windows failed me.
In the end I solved it like this:
1. Upload all .mer files using ME Transfer Utility
2. Restore first .mer file into FTView Studio and make a small visual edit
3. Create runtime from the edited file
4. Download to to HMI using ME Transfer Utility using the exact same filename
BUT VERY IMPORTANT deselect the "run application at startup" checkbox first
5. Reboot the HMI
What this does is updates that .mer file on the HMI
but does not change which file is run at startup. So, when the HMI boots up, if it contains the graphical edit you made, then the file you edited is the "live" one, the one that is being run by the HMI. If it doesn't, you can eliminate that .mer file as a possiblity and repeat steps 2 through 5 until you find it.
Once you're done, make sure to download all of the original .mer files back to the PVP so that it's left in the same state you found it.
Or, you know,
delete the redundant files from your god dang HMI's so that I don't waste half a day cleaning up behind you