When measuring current 3-phase 3-wire active power on the DA100, I can't set the scale so that I can drop the 250 V, 50 A input current down to 5 A for measurement.

Set the span and scale to ±30000 (ignoring the decimal point). In this case the range is 250 V, 5 A, so the span is in the range of ±1250 W. As you are dropping the current with the CT to 1/10, you have to increase the scale by 10 times. Therefore, the scale value will be within ±12500. If you set the left side of the scale to 0.0, you can't set the right side to ±3000.0. In this case, set the left to 0 and the right to ±12500.