Say an artist made a monochrome/lineart version as well as a full color version of an image. If the color image is already on the site, would any lineart or sketch versions be taken down as being incomplete or inferior to the color version?

Depends. Some sketchbook is pretty while some is not.

I would say that it's okay, for the same reason that we don't delete such existing posts when colorized/finalized ones are uploaded, but I could be completely wrong. Of course it has to meet minimum quality requirements.

(Funny that you asked one question in the title and the opposite question in the body, was confused for a moment.)

I would have no problem approving sketch versions by the same artist. They are not the same version and I do not see a trend of deleting them.