“Mixdown to one track” means that if you select multiple tracks or events, they’ll be mixed down into a single track. Normally you would get multiple rendered tracks. This option is useful for layered instruments and sound design. It’s not for converting files into mono.
Whether Render in Place will generate a stereo or a mono file depends on the plugin’s outputs. If your plugin doesn’t offer mono outputs, the quickest way to convert the rendered files to mono is to perform a batch export.
But if you need to keep the insert effects and other channel settings editable, you can go to the pool after rendering (Ctrl+P), select the files that you want to be in mono, then click “Convert Files”. Now create mono audio tracks below each rendered track, and drag the new mono files into them.
You can then transfer the channel settings from the rendered stereo track to the new mono track by opening the MixConsole and dragging the bottom of the old track (where its name is) into the new track. Automation will have to be copy and pasted manually.