The primary effect of thrust line offset is pretty simple and you don't need to know very much if anything about spiral sliptreams, vortices, etc.
Assume that the thrust is applied along a line, called the thrust vector, that is right along the engine shaft. If this line doesn't go through the CG (in all three dimensions), it applies a torque to the airplane. In this case, we are talking about yaw torque due to the engine offset to the right or left. It works in pitch too- adjusting the upthrust or downthrust changes the torque around pitch. If it goes straight through the CG, it applies not torque.
The in-flight thrust averages about 2-2.25 lb, and probably goes as low as 1 lb and as much as 3-3.5 lbs on a typical stunt plane and more variation in the wind. The torque is the thrust times the distance the line misses the CG, so if your thrust line misses the CG by an inch, you might get 2 in-lb. Not a huge amount but not trivial, either.
I think what you more-or-less want is the projected thrust line to go through the CG in the "plan view", to create no net yaw torque, and no variation in the torque when the thrust changes. That's why you want very little engine offset on airplanes with normal amounts of asymmetry, and a lot of right thrust on airplanes like the All-American Sr. with grossly excessive asymmetry. The asymmetry sets the lateral CG location, because you have to adjust the tip weight until the CG lines up with the lateral CP, so it sets the "0-torque" offset angle.
Up- or Down-thrust is more interesting. In most conventional layouts, the thrust line is already well above the CG, causing a nose-down pitch torque. Adding downthrust increases it, and you would need substantial up-thrust to get rid of this nose-down torque. I think you want a fair bit of nose-down torque to compensate for the nose-up torque from precession *with a conventional rotation*. With backwards rotation, you might need up thrust, or something like that, which may partially explain some of the odd characteristics and trim settings of reverse-rotation electrics.
Brett