ANSWER:

You are praying behind a person and not behind a masjid! What counts is the the aqeedah of the imam. If you don’t know his aqeedah, the default is that he is muslim until proven otherwise. 

Sheikh, the hanafi mosque near me has a chart of Allah’s names and “ya allah” in Arabic written. On the left side there is “ya Muhammed” with a chart of his names. They celebrate mawlid and do bidah like shab e barat and dua after prayer and the imam looks like a normal imam, can I pray behind them?

Prayer, Rulings on mosques | April 11, 2022 5:31 am
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I learnt that we go to the next post AFTER the Imam completes saying Allahu Akbar (or Sami’Allahu Liman Hamida). Please confirm. But most people in the Jamaat immediately bow or prostrate, etc. when the Imam STARTS saying Allahu Akbar. Should I go with the Jamaat, or wait till he pronounces completely?
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The mosques In this country pray taraweeh 4 Rakat with 2 tashahud and one tasleem and recite loudly in all rakahs am I sinful if I pray with them, if so do I get the reward for sins being forgiven by praying 11rakat of night prayer at home only for 1 hour or so ?