Player Behavior and The Grading System
Your grading system incentivizes bad behavior.
Selfish play in any of the most basic team games doesn't belong, yet players can earn S grades even when they lose. The win/loss result should determine the ceiling of one's grades. I suggest that S+ be the ceiling for a win result and C+ be the ceiling for a loss result. Players might work harder to win, given there's nothing in it for them if they lose.
To avoid wholesale abuse, you can change the penalty for inting to permaban, and include another bannable offense under 'report player' called 'throwing,' wherein a player may not have fed but did little or nothing to prevent the other team from winning. Other similar balancing measures in the penalties, all equally draconian, would be very easy to implement over time as you gain more feedback.