Why did Eve have to be in Adam before she was created?
Eve had to be in Adam so that Adam could be the federal head of the whole human race, men and women, making possible only one Redeemer. Otherwise, God would have had to provide a separate Savior and Redeemer for the female, which would be ridiculous.
Les teaches that Eve had to be in Adam even before her creation, not physically but conceptually, so that Adam could be the federal head of the whole human race. If she had not been, God would have had to provide a separate Savior and Redeemer for the female, rather than just one. Les illustrates this with angels: there is no redemption for fallen angels because each angel is a specific creation, so God would need as many Saviors as angels—ridiculous. Applied to humanity, Adam being the federal head of men and women means only one Redeemer is needed. This is why Genesis 1:27 can say God created him male and female even before Eve appears, and Genesis 5:2 recaps it.