the catching up to the juubi is a speed feat for her because the juubi was travelling so fast that it created a shockwave that knocked the other juubis down.
she didn't notice the juubis after the punch because she looked back at naruto and sasuke and her punch kicked up a ton of dust and debri, limiting her ability to see the juubis until they were close and the smoke cleared.
Sakura's fists are obviously the center of contact, like what other thing could possibly be the center of contact? If the ground is disturbed more infront of her than behind her, then it just means that her punch wasn't perfectly straight down, it was slightly angled forward.