What is the ruling on wearing a coat or overcoat that only reveals about a handspan and a half of the leg instead of a jilbab in winter, and praying in it, and is it permissible?

The obligation regarding clothing is to cover one's awrah (private parts). If the mentioned clothing is short, revealing the lower parts of the legs, then it does not fulfill the requirement of covering. It is not permissible for a woman to wear it in front of non-mahram men, because the entire body of a woman is considered awrah in front of them (with the exception of the face and hands according to some jurists). Likewise, it is not permissible for a woman to pray in it, because covering the awrah is a condition for the validity of prayer, and this includes covering the feet.