Yeah I think that player's minute from the previous week affect the GS as well.
I have never (In 3 seasons of playing) ever seen a player with game shape above proficient.
That's because the maximum GS a player can have is proficient (as you already know). I believe that as long as a player's GS is at strong or proficient, then his performance will be very good and you can get a lot from him (i mean his performance).
Then there's also always the fact that instead you could just improve your teams stamina which would let your players stay in the game longer and get their minutes easier, leading to better game shape.
Well there are usually 3 games per week, so it's really not that hard to get a player to play for whatever number of minutes in order to maintain his GS. But yes I see your point, with better stamina, it's easier to get your players to play more minutes, so that you don't have to get him to play 3 games every week, but instead 2 games per week etc.