If thermostat stays open then flow is reduced through the heater core. Pretty rare but possible senario. Will show up as erratic heater temp output.
To bleed you can use the garden hose trick. We used to do that on camry's and camira's, real pains to get air out. Do like you have already to check flow, just block off with thumb and keep it blocked(one end should do). Then fill radiator till fowinin out off engine heater outlets and quickly jam on the heater hoses (make sure clamps are already on the hoses:: yes, I have left them off before).