The data and histogram I see without Soft Proof on is reflecting output of what?
Melissa RGB. ProPhoto RGB with a different TRC than ProPhoto (2.2) BFD.
The thing about LR is that images and their edits are in a constant state of perpetuity until the edits are committed to export to a tiff or jpeg so while in LR with Soft Proof turned off the data reflected in the histogram and "%" numbers have very little connection to what's reflected in the preview with regard to measuring and defining the preview through HSL or RGB analysis. Do I have that correct?
When you soft proof, what you see is what you will get, visually and numerically after rendering through LR. Setup a soft proof for Epson 3880 Luster, that's what you get to see before a conversion to that output color space. Since you have asked LR to simulate the appearance, numbers and histogram (based on a RI as well), AND you decide at this point to edit
that data, the rational of the workflow process is now set for you: you apply output specific (or different from the master) edits so based on that new appearance and path, to a Proof Copy. ACR can't do that.
I never tried changing Soft Proof setting from ProPhotoRGB to another matrix profile to test to see a preview change, but if I understand you right (despite my doubts I'm able to), if I decide to Soft Proof to a printer profile with edits made using Soft Proof set to ProPhotoRGB on the "Make This The Proof" (I assume this means the master IS the proof) selection, the printer Soft Proof will only reflect the edits applied/encoded on the original master when Soft Proof was off and reflecting Linear with Adobe Default settings applied. Is that right?
The
Proof Copy workflow has nothing to do with the kinds of profiles, color space etc. You've got a master image shown to you one way in Melissa RGB. You switch that by setting a soft proof based on another color space. Either kind or profile, working space or output. The image on screen
might change, it might not (toggle from ProPhoto to Adobe RGB (1998) or even sRGB, depending on the image, it
might appear the same on-screen). The numbers and histogram will change. But you've told LR you want to work in another color space (fine) but further you now tell LR you want to apply an edit. Boom,
you must and should do this on a proof copy! The master is untouched just as if you duplicated an image in PS and applied adjustment layers for output specific edits on the copy.
All the other comments to follow you've made are immaterial to the above workflow you appeared to misunderstand and criticize:
And right behind it in annoyance is the subsequent warning dialog box after making the first edit asking if I want to make this the Proof or Proof Copy which I have no idea what the differences are.