How do sarcomeres coordinate contraction?
I'm answering question 1, I think it'd be better to post these questions separately.

Consider action and reaction: myosins (thick) filament is experiencing force F towards Z line.


If the muscle is acting on something that is movable (e.g. you're pulling on a drawer which isn't locked), the force F towards Z line is greater than the force that maintains your thick filaments in place, and they'll slide towards Z line. The speed at which they slide is limited by the rate at which myosin heads can detach once they have performed power-stroke.

If the drawer is locked, then force F cannot overcome the external force. This force however is normally lower than the force required to tear actin or z-line, it is limited by two molecular factors: myosin will stall and not effectuate power stroke under too strong an opposing force, and myosin heads  will detach after some time and relax the tension they have built up. This is how you get the maximum force under clamped conditions.