What is the ruling on praying behind an Imam who omits saying "ameen" aloud, does not raise his hands, lets his garment hang below his ankles, does not place his right hand over his left, and suffices with only one tasleem? And is it valid to pray behind an innovator? And can congregational prayer be established outside the mosque?

What you mentioned about the Imam abandoning some Sunan (saying 'Ameen' after Al-Fatiha, raising hands when saying 'Allahu Akbar', placing hands on the chest, the second Tasleem) does not invalidate the prayer, and the prayer of one who neglects them is valid. Your prayer behind an Imam whose innovation does not amount to disbelief is valid, and it is better than praying alone. This is the view of the Hanafis and Shafi'is. As for the Malikis and Hanbalis, they provided details on this matter.

The prayer of a disobedient person (fasiq) or an innovator (mubtadi') is valid, and the prayer of the congregant behind them is valid. However, it is disliked to pray behind them if they openly display their innovation or transgression, because they deserve chastisement until they repent.

The virtue of congregational prayer is achieved by the prayer of two or more people in any place. This is the view of the majority of scholars, with their agreement that praying in the mosque is superior. If it is possible to pray in congregation behind an Imam who adheres to the Sunnah or at home, that is preferable to having an innovative Imam.