To both of your questions: No.
The adapter is sold at the Apple web site, and has some clear statements on the
page:
This adapter supports iMac Pro and cannot be used with any other iMac or display.
The VESA mount iMac models are actually separate models, and don't ship with the normal stand.
I
think that the VESA adapter could be swapped out, but is a major surgical procedure - the stand, or the VESA adapter, are mounted from inside, and require removing all inside components to get at the mount screws (but don't assume that it is possible, as I am not even sure about that)
The new iMac Pro will take the adapter similar to VESA adapter plates on other displays - from the outside.
NOT useful, or "adaptable" on standard iMacs.
Maybe next year, when other iMacs shift to the newer case (thus making even RAM upgrades on "normal" 27-inch iMacs very challenging, if it is still possible at all)