To truly widen your shoulder you would Need to widen the clavicles AND the scapula
To widen the Scapula you would Need to make a split-cut on the green marked area (just like on a BSSO) to push the outer part of the Scapula outwards while also cutting the clavicles plus pushing them outwards
With this procedure it would be probably possible to extend the shoulders up to 4 Inch, you would also not Need to to cut into any muscle, you would only Need to stretch the muscle attachments which would be absolutely possible without losing to much muscle strength (of Course you could not become a top Tier powerlifter afterwards)
Of Course shoulder width isnt everything, it is also About the ribcage width, but from what i know it is easy to expand your ribcage (bonewise NOT muscle) through Training, Arnold Schwarzenegger did this with Pullovers i think along with correct breathing
From what i know it is even possible to widen the ribcage via surgery
Although a smaller ribcage will also create the Illusion of Broad shoulders
