I discovered a trick by accident - how to copy B&W settings only from one image to another. 1) create a clone variant of the image you want to make B&W. 2) copy the settings from your source B&W who's B&W settings you want to copy. 3) paste these settings on the clone variant you created. yes it will overwrite your existing adjustments. 4) copy the adjustments from the variant you cloned in step 1. 5) paste the settings onto the cloned variant. Steps 4 and 5 put back the adjustments you already made to the image and only leave the B&W settings you add to the source B&W image. Not perfect but a good start when you have lots of images taken under the same setup like a studio senior portrait session, etc.
Are you using the new Lightroom; the version that's the same (mostly) on desktop and mobile? Then you may have noticed that a few features are NOT actually on both platforms. The ones that get me the most are Copy Photos (ie duplicate photos) and Batch Paste. I've found a way around it… in a round about sort of way. Check it out!