Thrust line is exactly what it says it is. It's the line the prop pulls everything along. It's the line the jet engine pushes itself along. It basically a line that traces where the prop or jet would go if everything else wasn't along for the ride and encountering the air and it's effects. Of course, neither of those is possible. Except maybe for the GeeBee and the Rata.

Designers usually start with a specific airspeed to work from. It's usually the cruise speed since the airplane's performance (speed and range) for that is the most important. And they want the least drag (usually) at that speed, so they set the datum line of the fuse along that least drag line for the fuse and work from there. They could have the datum line down the center of the fuselage, or they could run it along in space below the whole pile of parts. It can be drawn anywhere, just so long as it's drawn the same on all the blueprints that need to match other blueprints. And they don't have to draw the datum line separate from the centerline, but often do to make it easier to place parts relative to each other.

Sometimes they called the datum line the water line, btw. Funny people, those airplane designers.