The "outer wing goes faster" explanation of yaw-roll coupling is not the whole explanation (so you are right about that). Dihedral is more important - the wing that goes forward when the aircraft yaws has a higher angle of attack and so creates more lift, which causes the roll. You might need a model to look at if you can't easily visualise what I mean.
The spiral slipstream does always hit the fin on the same side. It goes round the fuselage like a screw thread. Again you need to visualise it carefully. Of course if the fin was below the fuselage it would push on the other side. And some engine/propeller combinations rotate the other way round so the spiral slip stream is the other way.