I agree with the assessment that the 2018 is too stiff. Adding point weight may help weaken the shaft, but 2016's will likely tune much easier.
The other difference in the CEs and the 2018s may be shaft circumference. Depending on how the riser/shelf is cut, this could impact your tuning.
Four 4" feathers and three 5" feathers have almost the same amount of surface drag, so this probably would not account for major differences between your two shafts assuming the combined weight of the feathers is about equal on both arrows. That said, everything impacts tuning...and weight added to the tail of the shaft will have a stiffening effect.
You mention that the 2018 kicks left...does it also hit left at the target?
Claudia