Yes I think it would be good to dunk both of those plants in water. It will probably take a few treatments each a few days apart because the scale will hide down in the leaf axils, but this should still be easier than using chemicals. You can also just cut all of the leaves from the Zephyranthes because that is a bulb plant and the leaves will grow back.
If it's tricky to move the African violet around that's OK. It is very interesting to see them growing so well. I did not think they would grow with their roots underwater.
The palm must be a cat palm (Chamaedorea cataractarum).