The downthrust is essentially to correct for the offset of thrustline to center of drag.

with the engine below the wing, it tends to want to pull the nose up as you add power The downthrust acts to pull the nose down, and if the angle is right, they cancel out. Since each engine is a bit different, the angle is unlikely to be perfect... but 0 deg downthrust on a commn trainer, you could loop the plane just by manipulating the throttle.

The thrust line effect is used in some of the cheapie throttle/rudder electric planes to give a degree of "elevator" control. On those, appx 1/2 to 2/3 throttle is level, less is a slight dive (which due to tailplane incidence is liited from being too severe) and high throttle pulls the nose up. Thus... you get the ability to loop and do other maneuvers which nrmally require elevator.

The right-thrust is to offset torque and "P-Factor" the torque an effects of prop pitch related to line of flight tend to turn the plane to the left. Again, this effect is relate to throttle, and its actually possibl to have a plane controlled completly by throttle doing right-left turns, and climbs and dives. (they use this abilty in FF models, to get a left turn climb and right turn glide.)