In the absence of a response from Walker, I’ll attempt a short explanation. A digital image file mostly contains lots of numbers - for an RGB image all the RGB values and for a greyscale image all the gray values. I regard assign and convert as opposites.
If you [I]convert[/I] to an ICC then the numbers in the file change, but the appearance in Photoshop won’t (soft-proofing off). Why not? Because the numbers have changed, but so has their interpretation - the ICC.
If you [I]assign[/I] an image to an ICC then the numbers in the file [U]don’t[/U] change, but the appearance in Photoshop will change, because the interpretation of those numbers, the ICC, has changed.
So assigning an image to a soft-proofing ICC will result in Photoshop in effect giving you most of a soft-proof without turning on soft-proofing, because it will interpret those numbers in the file in much the same way as the printer will. (This is an over-simplification, but I think the general idea is correct.)
Perhaps we should wait and see if this helps the OP, rather than continue to hijack his thread. You can contact me off-forum Larry if you want further elaboration.