Louie B. Hydrick
In the time we are addressing, labor was comparatively inexpensive, and hand loading/unloading of many commodities was common. Logistics of returning empty coops made the use of separate coops financially unfeasible if not impractical.
I have a rather extensive treatise from the Charleston & Western Carolina Railroad about the difficulty of getting shippers to use consistent produce baskets to maximize the use of space in various reefers and produce (aka watermelon) cars. Produce remains still during shipment, and does not need a lid, so the baskets could "nest". Not so with poultry.
A 24" passageway would be snug, but not impractical for the average person in that time frame. We as a nation have definitely gotten fatter; however, there are still homes built today with one or more 24" doors, mostly to closets.
I imagine that hand loading chickens would be like most other manual labor on the farm, experience would breed expertise.