Does one receive the reward for congregational prayer if he prayed Maghrib in the mosque, then remembered that he had prayed without ablution, and so repeated his prayer at home after performing ablution?

You did well by repeating the prayer after realizing the error in purity, and you will be rewarded for the repetition. You are also hoped to receive the reward for praying in congregation. The scholars have stated that whoever intends to perform an act but is unable to complete it due to an excuse will receive the full reward for that act. They cite various hadiths, including the Prophet Muhammad's (peace be upon him) statement regarding those who stayed behind from Tabuk: "Indeed, there are people in Medina who, no matter what distance you traveled or what valley you crossed, were with you [in reward], and they are in Medina; an excuse detained them." Similarly, there is the hadith (definition): "When a servant falls ill or travels, what he used to do when he was healthy and resident is recorded for him." Sheikh al-Islam mentioned that this is a principle of Islamic law: whoever resolves to perform an action decisively and does what he is capable of from it, is considered like the one who performed it. An example of this is someone who purifies himself and goes to the mosque with the intention of joining the congregation, but finds that he has missed it; he will still be credited with the reward of congregational prayer.