Is the ablution considered valid if a woman wets part of her hair, while the rest of her body has already been washed, due to the difficulty of water reaching all of her curly hair, taking into account the religious importance of adornment for her husband? And must the prayers missed in the past, for which the obligatory acts were performed with such a ghusl, be repeated?

Your ritual purification from major ritual impurity (janabah (definition)) is invalid if you do not wash your entire head; it is obligatory for you to wash your entire body. There is a difference of opinion regarding the obligation to re-perform the prayers you offered with that incomplete purification. The majority of scholars hold the view that re-performing them is obligatory. There is no great difficulty in combining correct ritual washing with beautifying oneself for one's husband. If correct washing conflicts with beautifying oneself by leaving the hair unwashed, then the correctness of the ritual bath must take precedence.