Is it permissible for the one praying alone to receive the reward for congregational prayer after the Imam's ablution becomes void during the prayer?

You and your brother will be credited with the full reward for congregational prayer because you intended to pray in congregation, but an impediment prevented its completion. The scholars have stated that whoever intends to perform a righteous deed and exerts their utmost effort, but then an impediment prevents them from completing it, will receive the full reward for the deed. They based this on the Hadith (definition) from Tabuk: "Indeed, in Medina are men who were with you in every journey you undertook and every valley you crossed; they were detained by an impediment." And on the Hadith: "If a servant falls ill or travels, what he used to do when healthy and at home will be written for him." Thus, if a Muslim resolves to perform a righteous deed but is unable to do so due to an impediment, the full reward will be written for him. This is what Sheikh Al-Islam Ibn Taymiyyah affirmed.