The valve blocks reverse flow completely until pilot pressure, usually taken from the opposite side of the circuit, forces it open. So it holds by default and only permits reverse flow when the operator is deliberately commanding movement in that direction.

Effectively perfectly. It seats positively rather than relying on a clearance, so leakage is near zero and a load will hold indefinitely without creeping. Where a cylinder simply has to stay exactly where it was left, this is the correct and cheapest way to achieve it.

Because a pilot-operated check is being used where a counterbalance valve is needed. On an overrunning load the valve opens, the load drops, pilot pressure collapses, the valve shuts, pressure rebuilds and it opens again - producing a cyclic juddering descent. It is the classic symptom of the wrong valve.

On loads that are held rather than lowered under control - clamps, stationary platforms, held tooling, and any cylinder that must not creep but is not lowered against gravity. For anything that overruns and must descend at a commanded rate, a counterbalance valve is required.

Two valves in one block, each piloted by the other side, so a cylinder is locked in both directions when the control valve is in neutral. It is the standard arrangement for holding a double-acting cylinder in position without creep in either direction.

Yes, for the same reason as any holding valve - a failure in the pipework between valve and cylinder would leave the load unheld. Manifold and port-mounted blocks exist for this, and mounting one remotely for convenience removes most of the protection it provides.

Yes - it determines how much pilot pressure is needed to open the valve against the held pressure. Too low a ratio and the pilot cannot open it; too high and it opens too readily and becomes unstable. Ratios are stated by the manufacturer and should be matched to the circuit.