direction of the radiation reaction force

It's too difficult for me to give a detailed explanation in this type of forum but generally speaking it is in contradiction to the acceleration, if different from the velocity vector, as in a curved path. It is actually more complicated than that as you are taking into account both the force itself and also the stress on the curved path of the electrical field, i.e. there are two components. One of the consequences of this is when the velocity is very low the force is at a right angle to the direction of motion and so provides no reactive force at all. I hope that helps a little. You really need to see this diagramed out to get into details.