For the holding your line question ... I think the Pervis Kilo video from worlds this year on you tube is the great example of how to hold your line. And if you notice he tosses his bike a bit as he accelerates as well ... I like that too ... stiff bike is bad for me. His bike touches the red line a few times and the black line more than not, but in general his bike is between the lines, not on the black. He is staying low, but not actively trying to hold his bike on the line and just seems to be worried about going forward more than anything.
To Hermes, the British coaches who had you "practice" on below the black lines is one thing, but I certainly hope they were not advocating that you need to ride that way in competition, even for a pursuit. A lot of coaches scare/ruin riders by having them practice stuff that is good to practice, but it is just that practice to make you better, not something to think about when racing and they forget to tell you that. Once you get into a race you need to be more fluid and use your training to help you, but not run your race as you would a training session. You don't have to try and race perfect like a robot, let the bike go and make it go forward and let your training and practice keep you where you need to be, but never fight the bike. I am not saying you said all this, but I think it is important to differentiate practice from race.