I'm no lawyer and maybe I'm a little stupid, in these matters, but, assuming the checks don't have any type of release on them, I see no reason why they can't cash them.
The letter indicated that the amount was only 11% of the amount due, and the balance will be coming, so it implies they will be paid the full amount. With no release and a letter indicating they will be paid the full amount, it doesn't appear that the players are giving up any future claims or rights.
I would be interested in hearing why they shouldn't cash the checks.