With case B, the plane will drift left if drag is greater than thrust and the plane is slowing down. IF the plane is traveling at constant airspeed (thrust = drag) then it should track straight down the strip if it's actually tracking that way already.

The reason for drift left with decreasing airspeed (more specifically thrust less than drag) is that drag vector can be broken up into two components, one aligned with the ground track and a 2nd one 90 deg to the left side of the ground track. The thrust vector (engines and a bit of gravity if you're descending) can be split the same two ways but at constant airspeed would be of equal and opposite magnitude.

If you reduce the thrust, this balance no longer exists and the plane will experience an acceleration (change in ground track) with respect the the magnitudes of those two components, ie the component normal to the strip will accelerate the plane at a certain rate to the left of the strip while the component aligned with the strip will accelerate the plane backwards with respect to the strip. The result is a curving groundtrack drifting left of the strip as you slow down but the heading of the plane won't change as it really only "feels" the drag pulling it directly backwards at an angle of 30deg to the strip.