P: LAB values in not reflect image colors from Selective Adjustments to Monochrome images

yes, a bit OT but a new virtual copy is no different than the first copy - the two can even switch places - and in this nondestructive environment the selection of a monchrome profile can simply be reconsidered in either one, and a full colour profile selected instead.

My own habit is to make a B&W treatment by lowering the saturation to 0 across all the HSL colour ranges, so am always conscious of dealing with a colour image even so (continues to use a colour profile as it happens). And when externally editing I will leave myself the full colour there, and defer any B&W treatment(s) to be only applied onto the image that returns into the Catalog from PS. For the most freedom to vary that B&W conversion.

Thanks @richardplondon . On the second question (which is now off-topic since the topic has moved!). Yes, I was aware of the post-input colour grading, and the process of colour-toning a B&W image is logical. 
I guess what was surprising for me is that creating a virtual copy of a colour image then converting the copy to B&W can still leave colour in the image.  I would have expected that the input would be the act of creating a virtual copy (to your point about the profile stripping the colour on input).
Thanks anyway.  I'm a little wiser.  

@Bob Somrak 

I note that until you close the Mask panel, the color does read in LrC but does not in CR. 

to the second question in the OP title, a monochrome profile strips off all hues on the input side (so, deriving from the imported file) and tells the Basic panel adjustments to treat the image in a B&W manner.

But this does not prevent non-neutral RGB values from arising during other postprocessing, that will show on the output side. Perhaps the clearest example of this is Grading, formerly named Split Toning, which could never have e.g. achieved a sepia tinted or cyanotype 'effect', if denied the ability to vary a monochrome image away from its neutral hue.

@Rikk Flohr: Photography 

Start with Color image

Select B&W

Add Sky Mask and use both Red and Green curves to make sky blue.  Then EXIT mask mode

With cursor over blue sky and histogram in RGB mode the values indicate a color blue

is under cursor

Switch histogram to Lab Color and with cursor over blue the A/B values are zero

Lab and RGB shoud both be indicating color but only RGB does.

@Rikk Flohr: Photography 

I can duplicate what the OP is showing.  If the histogram is set for RGB the color values are reflected correctly but in LAB the A/B values are always zero.

For Item 1: I cannot replicate - supplying color via Color Swatch or Tone Curve does represent as color in LAB mouse over.

For item 2: Colors applied via Local Adjustment tools are treated as overlays, not image data.  Many people use this to create warm or cool-toned monochromatic versions.