I got it. By taking away the mamumamu, you completely take away all penipeni related pregnancies. Since all alices use their peni peni but it results in stalk births, this would make sense
Normal sperm filling seems to work this way: if its emmited through the skin by rub-rub, the result is a stalk with babies; if it is emmitted by the Peni Peni, the result is a tummy pregnancy.
Alice in rape mode's sperm custard has a random pregnancy property, see post #3754.
An interesting series, but as others have pointed out, yukkuri are so variable when it comes to reproduction that this kit would only make a slight dent.