Your problem is that you are just desaturating the image, this will always provide very poor results. I Think what you will need to do is take a TIFF into the PSE editor, rather than using ACR, as I don't think the PSE version of ACR has the Monochrome conversion option with the eight channel mixer. Also it seems to me from the way you are describing things as if you are trying to apply edits to the RAW file in DPP, followed by ACR and then back to DPP. This is not actually possible, you would have to convert to a TIFF in order to do this, maybe you are and I missed it in the description of your workflow, but I did notice you mention desaturating colours twice.
Once you have converted the image to a TIFF and opened it in the PSE editor, you would want to use ideally the Channel Mixer set to Monochrome, or the Black and White filter (the Monochrome option in ACR works the same way if you have access to it.) All of these methods for converting to black and white allow you to change the way that the various different colours are converted to shades of grey. This effectively changes the spectral response of the conversion, in much the same way as using orthochromatic or panchromatic film would, as well as mimicking the use of various different coloured filters, Yellow, Orange, Red and Green were perhaps the most common, that would have been used with black and white film.
Changing the colour channel mix has a VERY VERY BIG EFFECT on the look of your monochrome conversion. Here is an example that I picked because it has a both very bright red and blue areas in it.
Colour image.
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© BigAl007 [SHARE LINK] THIS IS A LOW QUALITY PREVIEW. Please log in to see the good quality stuff. This is what you get if you simply desaturate it.
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© BigAl007 [SHARE LINK] THIS IS A LOW QUALITY PREVIEW. Please log in to see the good quality stuff. I'll have to go to a second part to show the effects of using the channel mixer.
Alan