For the plane drifting during stall: the prop spins and sends the air spiraling over the plane, and creates a spinning mass in the front of the plane. These conditions would cause the plane to fly off at some angle other than straight ahead. Arf planes typically have some right thrust built into the firewall to compensate. However, the amount of thrust offset you really need is dependent on speed and rpm, not a fixed value. As such, the plane will drift different amounts in different conditions, probably trimmed to fly straight at cruise. It is very noticeable in a stall turn with an abrupt change in rpm and velocity, as well as a birds eye view of the plane.

The options are to live with it, change the thrust angle to suit a different flight condition, or mix rudder to throttle.