Here how I see it:
At the infirmary, dawoon didn’t heard that they were faking the relationship between mina and jin (we did buuut it was not obvious since it’s a little line that indicates that, I agree). He just got the fact that they tried and Mina catched feelings and Jin not but mina could stay friend while Jin didn’t care and was okay to stay indifferent and friend with her. So Dawoon discovered that what he was afraid (losing jin because of romance) was not true because he gets that Jin faked dating her to protect him, that they always turned around the pot because they were afraid of the same thing and yet couldn’t stop loving and protecting each other.
Can someone explain the last episode to me. What he means by don't do again I didn't catch that very well maybe I stupid I don't know