Yeah it can take a while to think these things through but quick enough to write. The doodles took a little bit longer than my normal post too. This is pretty much in line with my other threads I've started (see signature).
That's exactly it, some coaches will teach certain skills better than others and all players learn at different speeds! Players would slowly get better during the season (hopefully!) but the coaches would put most of their time into progression in the offseason.
Hidden potential with vague progression/regression ratings means you don't know if you're player is a bust until the end of their rookie contract. A player could have high hidden potential but low progression value, so they take a while to get there (Morris Claiborne for example). A player with moderate potential but 99 progression rating would pretty much peak as a solid player after their rookie season and not get much better after that, but you don't know that until a few years in when they're not getting better.