You have far, far too much faith in Long. Pretty much every comment about a disappointing hitter regards Long turning them around. He is a good coach, but performance is probably more like 90% on the player and 10% on the coach. Marsh and Sosa have flaws that have been exposed over time. They both can improve, but they were never really plus hit tools. I have hopes for both but also realize they probably profile as below average starters - not the type I would extend early.
Stott is different. He can be at least average and maybe a tick better if his defense improves. And the floor of being a good defensive 2B with a good hit tool (for him a 700-750 OPS with a strong OBP component) is still a 3-4 WAR player even at 2B. That is why it is worth thinking about extending him for a discount.
Early extensions give you a 30%-50% discount. Once they hit arbitration though the discount is rarely much more than 10% (since you have had 2 more years to judge them - i.e. there is less risk so less discount). The older the player gets the more injury risk too regardless of when you extend them.
That is why the early extensions on 22-24 year olds generally have an excellent track record. Low injury risk because of age. And bigger discount because you are doing it early. We never got any sort of discount with Howard. That led to disaster when he hit free agency. If we had only locked him up for what they would have been huge dollars after his rookie season it would have been a huge win. An 8 year deal after his rookie season might have been for $200 million, but we would have gotten 6 good years and 2 injury filled seasons and not had to deal with the final 3 mediocre years of his injury dominated back end of his career. Sometimes going year to year and being transactional is not very smart.