This is covered in an article in the current issue of the Star Trek magazine. According to Norman Spinrad, Blish made those changes himself for unknown reasons -- there was never a draft of the script in which Decker's first name was "Brand," and certainly none where he lived at the end, because the whole idea was "Moby Dick in space" with Decker as Ahab.
Keep in mind that Blish wrote these at a time when novelizations weren't expected to be any more faithful to the original shows and movies than films based on books are faithful to their originals. It was perfectly acceptable back then to change the story when adapting it to prose, since after all there was no home video and few reruns so it was unlikely that anyone would be able to review the original to compare. These days, we demand much more fidelity from novelizations. (And actually it was Star Trek that started that. It was those same 79 episodes running over and over for years on end that enabled the viewers to gain intimate familiarity with the details of a TV show for the first time, which led to pressure on Blish and his wife J. A. Lawrence -- who adapted or co-adapted the later volumes under his name when he fell ill -- to make those later adaptations more authentic.)
Say it ain't so...nothing like that could EVER happen...even almost 15 years later! 
(Though it was hardly a new idea even during Trek times...witness the original origin of Lightning Lad!)
