When does the Wudu' of the 'Isha prayer for those with excuses become invalid? Is it at the advent of midnight, or upon the passing of two-thirds of the night, or something else?

Scholars have differed on the final time for the 'Isha prayer, and the more preponderant opinion is that it extends until the time of Fajr begins. As for one excused due to urinary incontinence (sals) or similar conditions, they perform ablution after the time for the prayer begins, and their purification remains valid unless another nullifier occurs, apart from their constant incontinence. With this ablution, they may perform as many voluntary prayers, circumambulations (tawaf), and touch the Mus'haf as they wish, until the true dawn appears. Ibn Qudamah said: "It is permissible for a woman with irregular vaginal bleeding (mustahadah) and those in similar situations to combine two prayers, make up missed prayers, and perform voluntary prayers until the time for the prayer expires."