The Halacha of a woman not remarrying within three months is, in fact, Derabanan”
And that is specifically for the point of knowing if the woman is pregnant with her husband’s child, whether she is divorced or widowed. So please tell me, why does a woman who is clearly in her post-childbearing years, or one who has had the removal of her reproductive organs, ALSO have to wait that same amount of time to get married? Why must a woman who was an agunah for decades, not be able to marry immediately (same as her ex-husband can), when she is finally freed, and clearly has not lived with him for WAY past those three months?
And DY, while you are absolutely correct that the halacha of no concurrent multiple husbands is “because Hashem said so,” and because she becomes an aishes ish, perhaps Hashem said so davka because of exactly what Bais Yaakov Maidel said.
Were women to be able to marry more than one man at a time and be an eishes anashim, it would be very difficult (certainly in olden days, when there were no DNA tests) to know who fathered her children. From the standpoint of yerusha, bechora, Kohein, Levi, Yisrael, etc. this would create a real mishmash. So Hashem in his infinite wisdom, gave us laws that preclude such a tangled web to unweave, and though we cannot know His reasons for sure, just as with the laws of kashrus, we most certainly can see the benefits of those laws.