>Hand brakes on locomotives and cars (subsequently applied in sufficient quantity as prescribed by operating rules) would have provided another static braking force at the wheels, effectively a parallel or backup brake system to hold the train in place when "bottled air" in the car reservoirs eventually bleeds off.

Handbrakes are merely a second means of applying the same set of brake shoes to the wheels, using the same system of foundation brake rigging. Translated, this means "same braking force, different source of power behind that force".

>It's been reported that handbrakes had been applied on the locos, not the cars, so apparently bottled air

Let's say "stored air pressure" here because "bottled air" always refers to the practice of closing the angle cocks at each end of a cut of cars, leaving the Brake Pipe pressurized after the locomotive(s) are cut off. "Bottling the air" is specifically prohibited by the Rules.

>"Leaky" (or any professional railroaders familiar with air brakes),

There are two forms of "leakage": Brake Pipe and Brake Cylinder. Assuming the system has been properly charged, Brake Pipe leakage will cause an increase in the power of a brake application (up to a certain limit). Brake Cylinder leakage will lessen it on whichever car(s) have leaks in the brake cylinder and/or the associated application apparatus in the control valve. Brake Cylinder leakage varies from car to car.

>how long might it take for typical freight cars (with proper COTS work) to eventually lose charge of air in each car's air reservoir?

ISTR the test standard is that a brake application's cylinder pressure must not drop below a certain value for 5 minutes, measured with the [Brake Pipe] pressure maintaining feature of the [locomotive] brake valve inoperative. For a car which has had a "proper" COTS, with the brake apparatus repaired by a top-flight air brake shop, an application can persist for several days.