Not that I completely disagree with you here but I do want to add my critique.
1.) while this is true, I think it is partially due to them being in a different system than other international players. Most guys taken internationally and subjected to the international rules get signed when they are 16 years old, a lot of these high profile cubans are much older when they sign. When Puig signed he was 21 years old and was 22 when he made his debut. Moncada is year older than Puig was when he made his debut. Jose Abreu was 27 when he made his MLB debut. Profar made the jump to the bigs in about the same time as Moncada, but was sent down for the next 2 years and has only made it back up this year as a 23 year old and is doing pretty mediocre. Jose Iglesias rushed through the minors in about 2 years but he was rushed for his defense and didn't really establish himself as a player until he was 23. Moncada is being rushed for the opposite reasons.
2. I don't think it really matters that he did or did not have a "true" position in Cuba. It's the concept of sticking a guy where you think he will play best, and you let him develop there. You don't screw with his head and make him learn new things when you want him to focus on his offensive game. If the team feels a guy can help the club and needs to switch positions he doesn't need the mental focus on developing an approach that he had before.
3. I doubt this, day one he was barely 20 years old and put up a .813 OPS in low A ball, he did have a .950 left handed....but again this was in LOW a ball. I don't think we can decipher how he would have handled making the jump up 4 levels at that point. I think he's ready from the left side now.
If anything, I think I'd be more on board with your argument if we were to say that the Sox should give him more time at 3B, but he's ready to close to ready and the club thinks he can help now. He can still go to the Arizona fall league and get reps in at 3B and get some time there in spring training as well. He could also spend a month in Pawtucket next year to polish up his game in the beginning of the year too.