Hi! In my opinion, I think you might be buying the Chanel flats a bit too small. Chanel definitely runs a half size small for me. In the RV flats, I am my normal size, but in Chanel flats, they are always a half size bigger. Chanel is the one brand that I take a half size bigger in any style... flats, sandals, pumps.
My friend bought a Chanel flat, and I was with her when she bought it. The fit was good to a tad bit snug, and I suggested she get the larger size. She wanted to stay with the smaller size (why are women so afraid to go up a size?). Anyway, she regrets it now, and all her other ballets are a half size larger. The smaller one squeezes the top of her feet, around the toes.
Maybe try a larger size in Chanel. And don't be afraid if the SA tells you it's too big. Many SAs have told me my ballets are too big as they have a tiny gap at the back near the heel. But I just say "I like them big and comfy". And honestly, they ARE comfy! I have walked around NYC all day in my ballets with no problem at all. The Chanel ballets are my go to shoes for total comfort.
My other opinion on breaking shoes in. I know SAs say that too and a lot of people feel shoes need to be broken in. I say shoes should fit good when you put them on, otherwise, they are not the right ones. Especially if they feel tight... it's because they are too small. When shoes "break in", what people really mean is that they stretch them. But honestly, if you look at people's shoes, you can definitely tell when people are wearing ill-fitting shoes that they had to break in. The shoe is all stretch out looking and it is usually around the toe part and the wide part, so now the shoe has lost it's shape and elegance. Anyway, that's my opinion ad something to think about. Shoes are beautiful in it's original shape.
I hope this was helpful. Grab another Chanel! I really love the look, elegant yet also so comfy.