2. The plot of the game is that the MC has X amount of time to impregnate Y number of women or he forfeits his soul. Losing one's soul in fiction generally results in death.
facepalm. No, that is not the plot at all.
The plot is the MC sold his soul in exchange for demonic magic powers and is now the agent of the succubus.
She does not want to eat his soul (or else she already would have), she wants him to do stuff to advance her agenda.
Her end goal being to subdue her sisters, the arch devils of the other sins. (you for example already subdue the arch fiend of gluttony with your mighty demonic cock)
why would the archfiend of lust need the soul of a rando mortal, when she can get power over the other archfiends?
Furthermore, the archfiend of gluttony is now under the influence of the MC, not that of the archfiend of lust.
The MC is a critically important middle man. If he dies, the link breaks and the influence over the archfiend of gluttony is lost.
Further-furthermore. MC later on signed a contract with an archangel as well.
So he has both demonic and angelic magics and blessings.
Which most likely shields his soul from being devoured anyways. (this one is a only a logical deduction rather than a certainty)