Jaymo,
While you are correct that many back the Strain Screw out to "lighten" up the Trigger, it is not the correct way to go. As it would have shipped from the Factory, the Strain Screw would have been fully tightened. If it were impossible to cock SA, then it wold not have left the Factory.
Over the years many a Strain Screw have been replaced with incorrect ones causing what you call "impossible to cock" syndrome. Those SHOULD be replaced with correct ones and if one wants to slightly reduce pull, a Strain Screw must be slightly shortened and then fully tightened again rather than left loose or backed out.
While jsfricks is correct about not the correct way to do a trigger job, sometimes a very slight shortening of the Strain Screw does help - with emphases on slight.
As an aside, I have noticed over 40 years that NOT ALL Strain Screw replacements sold from Brownells, Midway, etc. are all the same. I have seen and have some examples of not only different lengths, but different configurations. I've even received slightly different ones directly from the Factory over the years and that is WHY a REPLACEMENT Strain Screw might need to be properly sized for proper function. I would bet that even S&W has purchased their screws from many different vendors throughout the years creating variations.