Can someone explain Player Positions to me? Am I to understand that I have to pick one position for each player to play all season?

Absolutely not.  Their listed position is irrelevant.  The only thing that it matters for is which player role formula the game uses for that player to calculate their player rating.  If you still have your player settings for overall rating set to "use total" instead of "player roles" then it doesnt matter in any way.  As like always, the players can play any position on the court regardless of their listed position.

Well, it doesn't matter in gameplay, but it does reorganize your players on your team pages when sorted by position.  If you recruited a SF to play C, you can change his position to have him listed on your team pages with the other big men.

I view it as only relevant in deciding what position you want each player to be considered at for awards at the end of the season.  That's it.

That's what I figured. Thanks folks