Is it permissible to divorce one's wife after marriage due to a feeling of discomfort and aversion stemming from seeing her hair before marriage, and the inability to go back on that?

It is best for you to learn about your fiancée's hair through your mahrams. The most important aspect to consider in a woman is her religious devotion, as encouraged by the Prophet, peace and blessings be upon him. If the woman is religious, we advise you to keep her and treat her kindly, and do not divorce her merely because of her hair, especially since you have sought Allah's guidance through istikhara and married her. Divorce is permissible but is the most hated of permissible things to Allah. If cohabitation becomes impossible and you fear you might wrong her, then it is better to separate from her with kindness. Divorce is not always a benefit, but sometimes its benefit may outweigh other considerations.