Is what the questioner did—interrupting the prayer and alerting the congregants to its invalidity because the Imam skipped an entire verse from Al-Fatihah, and then praying alone—correct or incorrect?

The Imam should not insist on errors in recitation. The recitation of Al-Fatiha is a pillar without which the prayer is invalid. If the Imam intentionally omits a verse from it, it necessitates leaving him; the congregants then complete their prayer individually or appoint another Imam. It is permissible to interrupt the prayer and start anew if the Imam insists on the error. The Imam of the mosque must be diligent in leading the prayers himself or appointing someone who performs the prayer well.