We're really sorry about the frustrated experience while using Photomerge in Photoshop Elements 14, that's definitely shouldn't have happened. Let us help make it right.
I'd recommend that you restore the app preferneces to default and let us know if that helps:
Preference settings control how Photoshop Elements Editor displays images, cursors, and transparencies, saves files, uses plug‑ins and scratch disks, and so on. If the application exhibits unexpected behavior, the preferences file could be damaged. You can restore all preferences to their defaults.
Note: Deleting the preferences file is an action that cannot be undone. Do one of the following:
Press and hold Alt+Control+Shift (Mac: Option+Command+Shift) immediately after Photoshop Elements begins launching. Click Yes to delete the Adobe Photoshop Elements settings file.
Go to Edit > Preferences (Mac: Photoshop Elements > Preferences > General), click the Reset Preferences on next launch button, and then click OK. When you restart Adobe Photoshop Elements, all preferences are reset to default settings.
A new preferences file is created the next time you start Photoshop Elements Editor. For information on a specific preference option, search for the preference name in Help.
"Can't edit a photomerge file in Elements 14 after saving and closing the file then reopening to complete more work."
Do you mean you want to save your photomerge as a 'project' in the .pse format? Today, if you save your photomerge in psd or tiff layered format, you can continue editing to a certain extent, but you can't re-open the source files to process again with different parameters. I prefer to start the photomerge from the organizer and to define my source files with a stack, an album or a keyword.