The shell theorem applies to a hollow, symmetrical sphere. The universe is not a hollow, symmetrical sphere.
All objects within your arbitrary sphere are subject to gravitational force. The shell theorem does not apply.
Gravity doesn't match expansion, it totally overwhelms it, out to a distance of about 200 million lys.
You're having difficulty grasping the concept, which, quite frankly, seems to be quite simple.
Gravity diminishes with distance, and expansion doesn't. At some point, gravity and expansion balance each other. Further than that, gravity diminishes to less than expansion, and expansion takes over. It's not that complicated an idea.