First of all, there is no such thing as aerodynamic dihedral. The direction of lift - up, down, or none - has nothing to do with dihedral effect. Wing, or horizontal tail dihedral adds lateral stability in the form of C_l_beta (or the derivative of rolling moment coefficient with respect to sideslip). When the a wing or h. tail has dihedral and is subjected to a sideslip, the lateral moving air creates a normal force on the wings. There is a component of this normal force upward on the right wing and downward on the left for dihedral and the opposite for for anhedral assuming the sideslip is positive (coming from the right). Therefore, the force couple created by dihedral will create a rolling moment to the left (away from sideslip - stabilizing), while anhedral will create a rolling moment to the right (into sideslip - destabilizing).

When I say that anhedral is always destabilizing, I mean in the lateral mode. As I said before, you could argue that adding anhedral increase the dutch-roll damping which increases directional stability, but that is always secondary to lateral stability.

And in terms of whether a horizontal tail creates a force upward or downward. For a longitudinally stable aircraft (c.g. is ahead of aircraft aerdynamic center) the horizontal tail will ALWAYS carry a downward force in straight and level flight.

In terms of stick fixed vs. stick free, you are correct in terms of stick-fixed being more stable longitudinally and directionally, but that doesn't mean that dutch roll can be ignored. Dutch roll is however much less restrictive in r/c aircraft because there are no passengers on board to get sick.