way you think is right, but implementation is as you might guess, wrong.
You should develop 2D in Unreal with 3D in mind. For example, check Third Person Template. Mesh got one Capsule Commponent, but also has a Physics Asset. Capsule is what we moving and mesh itself is connected to it and Physics Asset takes care of what happens if a linetrace hits mesh etc.
So what you need to do is having a Box Component in your Character Blueprint (Or Capsule if you desire, I like Box) that you handle all movement and make box block ground, sidewalls etc, so that will fix your 1.Box problem.
About other Boxes, unfortunately it won’t work that way either. It is true that you can’t have separate collision responses. For that you would have to have 3 Flipbook components with each of them containing each boxes. As you understand, that’s literally a hell to prepare.
There are a few ways you can use to make a character use “Get Hit to Head” / “Get Hit to Stomach” / “Get Hit to Legs” animation. After hitbox connects (BoxTrace to be exact in Unreal 4 terms), you can get Impact Point, check if it is closer to legs, stomach or head.
My way of handling is easiest one. Enemy determines which animation Reciever should play. So I specify that attack is aimed at stomach, so whenever attack connects, it always makes Reciever play Stomach hit Animation.
In end it’s all about chosing your posion.
Good luck.