Referring to the question of coaching, I have a very stark example of what inferior coaching can do to a prospect. Javier Bournigal is not a stud, but he is a plus-glove SS worthy of an ML roster spot. I acquired him in a trade many seasons ago from an owner whose fielding coach was atrocious:
as you can see, Bournigal made almost no advance in fielding skills between season 14 and 16. Only after I acquired him, near the end of season 16, did he start to improve. And improve he did! He has since gained 20 points in Glove and 18 points in Arm Accuracy. What I cannot answer is whether once he got the right coaching, he reached his ceiling, or whether some potential gains were left on the table because of his lackluster coaching those first few seasons.