Does Wii AR 16:9 + backend AR 4:3 produce the same effect in other 4:3 games for you?
I think what is happening is the game is outputting 16:9 with black bars when dolphin reports 16:9 instead of a 4:3 image. Which is why it's not getting scaled properly by dolphin in wii 16:9 mode.
My guess is that the game (Residen Evil Remake here) produces some kind of letterbox. Black bars on left and right are not just an empty space, they are really drawn by the game to make a solid 16:9 image.
Meanwhile Residen Evil Zero doesn't draw black bars and just stretches the image to fit widescreen so using backend 4:3 neutralizes this effect and leads to a correct undistorted image in this case.
While I was picking words you made the same assumption
If this is correct behavior for the game then there is nothing we can do about it. The Wii AR isn't editable per-game because it shouldn't need to be. I think you have managed to find the one scenario where it would help. Normally setting wii AR to 16:9 or 4:3 should not effect the game other than that the game shouldn't output 16:9 when it's set to 4:3.
He's trying to get proper 4:3 output.