Why are you so set on trying to logically analyze what is clearly a silly, illogical gag 4koma?
Anyone else who looks at this, or anything else by this author, can clearly tell it's not supposed to make sense.
Even having a "theory" shows that you're spending way too much time thinking about it, and talking down to other people about it just makes you look pretentious, not smart.
And yes, I made an account just to make this comment.
How does this pairing even exist? They hate each other, especially after what happened at the end. All Annie wanted to do was climb the wall but NOOO Mikasa had to cut her down.