No Dual mono option for voiceover

There must be something else going on to have mono audio only playing from one speaker. Have you tried resetting FCP Prefs? It's an easy thing to try when inexplicable things start happening in FCP. Hold the option and command keys while launching FCP, then choose Delete Preferences... It will open the default Untitled library. Re-open the library you were working on, set the preferences back to the way you like (background rendering off, leave files in place, etc.) and see if the issue has been resolved.

The recording is one mono channel.

I'd question why dual mono? You're not planning to pan them left and right, I hope. That doesn't give you stereo separation, and can introduce phasing issues. If it's mono, it's mono, why change it?

Then change the Pan parameter, simple.

Just because it is mono does not mean it plays on only one side. Are you sure that is what's happening?

Your screenshot shows it set to Left. Set it Mono or Center.

Because it’sa stereo recording. It can either be stereo so the channels are controlled together, put dual mono so the channels can be controlled independently.