And it was a big success at the time. Not counting movie novelizations, I believe Yesterday's Son was the first Trek novel to hit the New York Times Bestseller List.
No oversight at all. Pocket's first original TOS novel, "The Entropy Effect" gained huge amount of free publicity (and fan furore) by killing off Kirk. The editor probably had to have a good pitch ready to fight for Crispin's storyline. And yeah, it made the New York Times Bestseller List, thus vindicating that faith. And the "sandbox" still looked normal at the end of the novel. All the tie-in manuscripts were vetted by the Star Trek Office at Paramount. We know that Susan Sackett provided guidance on the Bantam novels, and probably the earliest Pocket novels as well. Richard Arnold was more critical of perceived liberties taken by authors. Of course, with the rise of TNG, and every month adding four new canonical (aired) stories to the timeline, tightening up what was and wasn't permitted for the tie-ins became an efficient option for 1989.