Should Maghrib prayer be repeated if it becomes clear that the time for Isha has begun before or during its performance? And is it permissible to pray after the obligatory Subh prayer?

If someone prays Maghrib believing it is within its time, and then it becomes clear to him that he prayed it after the time for 'Isha' had begun, his prayer is valid and he is not required to make it up. This is because the intention of performing a prayer at its due time serves in place of the intention of making up a missed prayer, especially when he was unaware, regardless of whether the time for 'Isha' began before or during his prayer. Al-Nawawi and Ibn Muflih mentioned that making up a prayer is valid with the intention of performing it at its due time, and vice versa, when the reality turns out to be contrary to one's assumption.

It is not permissible to intentionally delay a prayer beyond its prescribed time without a legitimate Shar'i excuse. Whoever does so has committed a grave sin and must repent. The best of deeds is to perform prayer at its appointed time.

As for voluntary prayers after the Fajr prayer, they are disliked if they are without a specific reason. Scholars have differed on their permissibility if they have a reason, as well as regarding making up missed prayers during times when prayer is prohibited.