The time Shika caught Garaa with his shadow and was hurt because Naruto punched Garaa, is easy to explain.
In fact Shika did it himself. He said to Naruto that he does the same motion that Garaa does.
That's why he was hurt.
Not from the punch itself, just from the rapid motion resulting from it.
So he could've hurt himself at the time with Kin. But not because she hit her head on the wall.
Only from leaning backwards, if Shikas back couldn't deal with it.
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"