Is not performing the 'Isha prayer after purification from menstruation, but before dawn, considered a sin, or is relying on the opinion that its time ends at midnight a strong basis, especially in the presence of obsessive-compulsive disorder?

The time for the 'Isha prayer extends until dawn. Its preferred time is until midnight, and what comes after that is a time of necessity. A menstruating woman, if she becomes pure before dawn, must perform ghusl and pray Maghrib and 'Isha. It is permissible for a person afflicted with waswasa (obsessive doubts) to adopt the easier opinion. If you prayed 'Isha after midnight, you have fulfilled your obligation, but one should not intentionally delay it until this time. Do not switch between different opinions; rather, follow someone you trust and strive against these whispers (waswasa).