Overall and potential ratings are a construction from OSA or your scout. They do not affect player performance and can be very misleading (especially with pitchers). You need to look at the "game" ratings (i.e. contact, gap, power, eye, avoid k, etc.) AND at the stats (and age, and injury proneness, and personality, etc.) to correctly asses a player.
And when I say stats, I don't mean only counting stats, but more advanced stats too. Every stat site has a primer on crucial stats (woba, war, wpa, etc.). I'd suggest taking a look at those to get a feel to how they could help you assessing your players.
Prospects need a lot of time to develop and bust out in baseball more than in any other sport and the game correctly models that imho.
I have found that maxing the player dev budget, when possible, is a bonus. Less so with scouting. Experiment. Ratings for overall performance and potential are created by OSA or your scout. They are incredibly deceptive and have no bearing on how well players perform (especially with pitchers). To accurately evaluate a player, you must take into account both the metrics (such as age, injury propensity, and personality) AND the "game" ratings (such as contact, gap, power, eye, avoid k,
https://12bett.in/ etc.).
The game devs say that yes coaches impact players. How much and in what way ? That's a mystery. It's also pretty much a mystery in real life too, so all you can go is hire and retain the best coaches you can and make sure they get along with your players.
They do and they do not. If you have scouting enabled, you need to remember that the ratings presented come from the lense of your scout (like in real life) and reflects his biases and the budget alloted to scouting. This is why you need to look at stats too (and other factors) to correctly asses a player. Depending on the rating scale you are using, two players could have the same rating in a category and perform differently due to rounding too.
Momentum does play a role in overall performance, but I doubt the game engine models "tanking", unless the player actually makes an effort to tank (i.e. play backups, bench starters, etc.).
You need to realize that baseball is very high variance (just like poker) and is probably higher variance than any other sport due to how hard pitching and hitting a baseball are. You could be holding all the best cards (or players) and still get a diffenrent result than expected.
I hope this helps.