The really important point that I don't think is emphasized enough is that when using a derate, you risk a loss of control when pushing up the power beyond the derate limit in an asymmetrical situation. When using a derate WITH an assumed temperature, you theoretically have additional power available to you up to the derate limit but unless you actually know what that limit is, you're playing with fire. Most carriers that I'm familiar with do not have procedures for calculating the actual derate limit N1/EPR/TPR, etc..
Yup, that is why boeing suggests that one does not increase thrust during a derate take off until, in the opinion of the captain, terrain contact is imminent. After all, the take off performance is calculated with the reduced thrust level and the obstacle situation of the runway in question. There is no need to increase thrust if all that fails is just the one engine.