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 convert 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 assign an image to an ICC then the numbers in the file don't 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.