have only one mic input into the Scarlett 2i2. When I add a new Stereo track and record onto it, only the Left channel gets recorded; the Scarlett has no Right channel signal to output
Yes, that’s to be expected. But, the signal won’t be limited.
After recording you can click the little drop-down arrow to the left of the waveform and select Split Stereo to Mono. Then you can click the little “X” to the left of the silent right channel to delete it.
don’t recall having the -6db limit problem before recently, nor have heard about such a problem from others.
It’s a common issue. I don’t know if it’s Audacity or the drivers but there’s a very good reason for it -
Let’s say you’re recording at full-volume from one microphone in mono with your stereo interface. Now, your back-up singer comes-in on the 2nd mic. The two channels mixed to mono can clip (distort) without triggering the clipping indicators on your interface, so you may not notice until after if it’s too late. Audacity leaves headroom for the 2nd channel. So with both channels recording in mono, the digital file won’t be clipped as long as the clipping indicators on the interface don’t come on.
FYI - Peak levels of -6dB or less are not a big deal with digital recording, especially if you’re recording at 24-bits. You can simply boost the levels after recording. Pros often record at -12 to -18dB. (In the old days of analog tape you needed a hot signal to overcome tape noise, but there is no tape noise with digital. 
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