What is the ruling on praying in congregation behind people who do not adhere to the Sunnah, such as performing the Maghrib prayer immediately after the Adhan and the Imam starting with Al-Fatiha after the opening Takbir without a pause for supplication, and what should be done to preserve the congregation despite the difficulty of going to another mosque?

There is no impediment to following the mentioned Imam and praying in the mentioned mosque, because supplication between the call to prayer and the iqama is recommended, and prayer is not invalidated by omitting it. Similarly, the opening supplication (du'a al-istiftah) is a Sunnah, and prayer is not invalidated by omitting it. It is possible that the Imam prays according to what is prescribed in the Maliki madhhab regarding hastening the Maghrib prayer and the non-legitimacy of the opening supplication in obligatory prayers. The rule is that whoever's prayer is valid for himself, it is valid to follow him. If you wish to perform prayer in a distant mosque despite the hardship, there is greater reward in that, but it is not obligatory to bypass a nearby mosque for a distant one.