The reason why you still do not understand the significance of the headroom is because you won't do what I recommended all the way back in the first handful of posts in the original thread. Run a simulation that plots the output characteristic of the opamp so that you can get a feel for how it behaves and what it will and won't do.

As shown here the output voltage cannot exceed 11V, and the current will not exceed 24mA. By referring to this I can see that the maximum current swing of the op amp in simulation with a 12V source is 11V. Am i correct in saying this is what you mean by getting a feel for it?

 

You might consider ways to control the bypass transistor that do not require the output voltage of the opamp to go that high.