You take the Landing Climb Limit weight from Performance Dispatch 22.3 and ADD the weight of the center tank fuel (because this fuel can be jettisoned) and 2,450 kg (the fuel that you would burn returning to the airport). This number is an additional restriction to your maximum takeoff weight, so you compare this weight to the regular runway-limited and climb-limited maximum takeoff weights and use the lowest as your operational maximum takeoff weight.
You are correct that you also have to consider the fuel scavenge system inop and need to consider the last bit of center tank fuel unusable and lower the maximum zero fuel weight . The MEL doesn’t make a distinction between one or both jettison pumps inop, so you need to use the same performance penalties in both cases.