It was already scientifically proven in 2016 that Mammillaria geminata is the older name. And M. karwinskiana does not fit the plants that we identify today under M. karwinskiana due to various characteristics such as reddish flowers see: http://www.mammillaria.eu/Probeartikel/Probeartikel_4_2016.pdf

Mammillaria nejapensis belongs now to Mammillaria geminata see aswell Cactusworld (2020): 38(4): 315-320.