1) Whether a jailbroken phone will be tethered has nothing to do with the firmware version and everything to do with the iBoot version. If a phone was untethered before 3.1.3, it should be untethered under any theoretical 3.1.3 jailbreak.
2) There is currently no way to jailbreak a 3GS on 3.1.3, but iPhone 3G's and 2G iPhones can be jailbroken using redsn0w.