Why does the Fall need to happen before conception/birth of Cain?
The offspring of Adam are not sinful only because they were concieved in sin or committed acts of sin.
The offspring of Adam are sinful because they participated in the sin comitted by their federal head/representative, Adam, thus breaking the covenant of works.
If Eve and Cain did not sin, and if Cain was not concieved in sin, they would still have participated in the sin of Adam due to his Federal Headship.