Skating in seniors nationally does not preclude you from going to junior internationals. Karen Chen can still do junior worlds and junior events next year if she wants to, though I won't be surprised if she makes the leap to the senior grand prix as soon as she can. Will she be old enough for seniors next year? If so, she might have chosen to skate the senior event with the hopes of at least getting Skate America next year.
Mirai's skate was heartbreaking and yet I have to say that I totally respect her guts. Few skaters could land so many jumps on a knee that felt broken.