胸 and 背中: the front and back of the body
Read attachment points and visible sides in expressions for the chest and back.
A location at the front
胸, read むね, means chest. 胸に名札を付けました。 says that someone attached a name tag at their chest.
胸の前で手を合わせました。 says that someone brought their hands together in front of their chest. 胸の前 locates the action; it does not place the hands inside the chest.
A location at the back
背中, read せなか, means back. 背中に荷物を背負いました。 means carrying luggage on one's back.
背中を壁につけて立ちました。 describes standing with one's back against a wall. Do not turn it into a statement about looking at the wall from behind.
What is visible and what is felt
友人の背中だけ見えました。 says that only the friend's back was visible. Adding that the face was visible violates the restriction in だけ.
胸がいっぱいになりました。 can express becoming overwhelmed with emotion in context. Do not read every occurrence of 胸 as physically filling the chest with objects.
Does 友人の背中だけ見えました say that the friend's face was visible too?
Show the answer and why
No. It limits what was visible to the back alone.