The whole point of UNIWB is avoid clipping the RAW data, something which is pretty much irreversible.

You ask what the point of this is, since you obviously don't want to use the UNIWB setting in your final image, and hence the red and blue channels will be clipped when applying a standard white balance. However, during RAW conversion you can reduce the exposure of the entire image to prevent any clipping whilst still enjoying a neutral white balance and correct colours.

The histogram on your camera is designed to warn you of exposure problems, it is not a tool to set the final output exposure level, that is a matter of taste and should be done by eye (on calibrated equipment).

Clipping in the final image is not as bad as clipping the original capture. You may decide that an amount of clipping is acceptable if you really want a certain exposure level. You may loose detail but that's the trade-off you have to make. But if you clip the RAW data and you decide you don't want that exposure level, then it's too late.

I don't know enough about how the white balance is applied during in camera RAW conversion to say for sure that the brightness of a channel is always increased, never decreased, however in general there is usually 1 stop more detail in the RAW file than in the camera JPEG. This means unless the "exposure factor" is less than 0.5, which seems unlikely, the camera histogram will never give you a false negative for RAW clipping.

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Ultimately UNIWB is a tool for determining the maximum exposure possible (with the implicit assumption that you pull this back in post) which carries the principal benefit of lower noise (and slower shutter speeds/faster apertures if required). My advice in these situations is simply to bracket your exposures whenever you have the storage space to do so, that way almost any metering/clipping problems you might experience simply go away.