You can. But should not. If fundal massage does not cause your uterus to clamp down, you could potentially bleed heavily, and require a transfusion or worse.
Usually and yes. Pitocin (oxytocin) is almost always used after a cesarean delivery to insure that the uterus stays contracted and minimize blood loss. Nursing causes the body to produce it's own Pitocin (oxytocin) which will also cause the uterus to contract and decrease bleeding.