What is the meaning of "对着／对上／对不上" in this pattern "Subj. + 对着／对上／对不上"？
槍口對著人 means that let muzzle of the gun face a person. Here, 對著, means face a particular things. It is apply on 對著鏡子笑 too. So you face a mirror and laugh.

對不上 means fail to match correctly. In examples you provide, the two accounting cannot match correctly. Let me introduce other example, you can say 這兩片拼圖對不上. In this example, it show that two pieces of jigsaws failed to match.

If they can match correctly, you can say 這兩本帳對得上 or 這兩片拼圖對得上. It substitute 不 with 得. So the negative meaning is removed.