I will try and help you as best as I can since I have done a lot of long term summing with this years version while waiting for OSFM to drop. I am currently running a padres franchise with OSFM so I will use his as an example.
First of all the game will always value A and B potentials over C and D. This means the player has a higher ceiling. Try and use the overall number (let's just say an 83 potential) as a baseline of where any given players overall will most likely be capped. Can they get better than this ceiling? Absolutely. But probably not by much. To answer your question about dynamic potentials, they are still dynamic in this years iteration. Meaning they can increase or decrease but believe me when I tell you this seems to be completely random. With a diamondbacks franchise I ran, I had Jake lamb tie for second in the league in home runs with 36, finish 5th in RBI's with 99 and bat around .260 and he took a potential hit even after making an all star team. While Brandon drug hit .234 with 16 HR and 57 RBI's and saw a potential increase.
A and B potential is not guaranteed to hit their ceiling just because it is sett higher. In my padres franchise I have minor league C rate prospects improving way faster than hunter renfroe who is a high 97 potential. While dynamic potential increases/decreases seem to be random, progression seems to be largely production based this year (this will lead me into your third question in just a moment). This does pose a potential problem however. From the short testing I have done it seems that the minor league production progression is based on the same curve as MLB production progression. The problem is most prospects simply do not have the attributes to replicate here numbers even though they are facing way lesser pitching competition. I have sinned 4 franchise seasons and have only seen 1 20+ home run season in triple A from a player that was rated 73 overall and could have been on my MLB roster. This seems to make the high potential prospects with low overalls flounder. My C potential prospects with high overalls seem to progress quicker and greater than my high potential low overall prospects. Sure I could stick them in single A but win only 90 players available on my roster I would not be able to fill out a AAA or AA lineup.
Now to your third question. It doesn't really matter where you play your prospects in the field. Let's say your stud 3B prospect has a secondary position of 2nd base, it is more Han fine to play him there. However, since the progression system is largely based seemingly on production this year you do want to make sure your studs bat 1-5 in the order since they will generally see more at bats and generate more production.
I hope this helps. Forgive any spelling errors as I typed this from iPhone with auto correct on