Oh, so you do NOT leave your liner within your frame once the new cable is in?
I've worked plenty on the P2 and internal cable routing. The answer to your last question is really "it depends".
My whole purpose of using the liner is to make the process of putting cables through one end and out the other. Almost by definition of this need, it means that the liner needs to be longer and exit out both ends of the frame. As a result, it's too long once the housing is added back in.
For the front derailleur, you can make a full length of liner work since there is a pretty big gap between the cable leaving the BB and extending up to the FD. In this case, I just extend the liner all the way up to the front derailleur. When I want to swap cables, I just slide the liner closer to the bottom bracket until it pokes out at the down tube.
Also, a problem with liners is that they tend to 'kink' making it more difficult to run cable in tricky places, like a bottom bracket. So, even when using for an FD, it might not be so easy.
My typical solution is just to slide on the liner to the end of the cable as I'm pulling old cables out. As the cable goes out, the liner goes in. For each cable, I also use two liners. One from the derailleur to the BB, and then another from the BB out to the downtube exit. Works every time. As the new cable goes in, I just pull the liner out.
Another point of note. Magnets are great. Another very useful tool is a paperclip bent in the shape of a hook to reach into those small spaces and help guide the cable out. ...that, and a headlamp to see what you're reaching for.