Why do you need to access the roof to attach at each corner?

A container is 2.6m high, so using a low hop-up (say 0.6-1.0 high) you can easily reach each corner from the ground. So... crane operator hangs chains over one corner, using hop-up you connect one leg to that corner. Crane slews across to hang remaining three legs over next corner, attach one using hop-up. Slew to hang over next corner, connect....repeat. Occasionally one of the hooks drags free whilst connecting other corners, so check all four before hoisting.

Complies with the work at height hierachy of avoiding work at height in preference to preventing falls

In certain circumstances what you suggest would be ok, but in most cases's if access is required only to hook and unhook why cant you use a ladder, as its short duration work, as long as the ladder is footed and 3 points of contact can be maintained etc etc. Why over complicate by adding harness lanyard retractable reel, which in turn requires maintenance,training etc.

I have a little experience of containers.

Could you employ a forklift truck to lift the containers? (The bigger version for containers).

OR

Use the PLS ? (Palletised Logistics System -- a copy of the British Broughton system). You can buy second hand from various outlets.

link...

While out in the middle east i came across this method

I hadn't seen it before and was a revelation in terms of speed
I'm not sure how good it is with a container with a high centre of gravity. Dont see it over here that much as the old single hi-ab method is most popular.

Using a ladder to access the top of the container is pretty well the accepted method for deliveries of containers, steel stores and cabins. Trained operative. Short duration. Used for access only. Footed (?difficult for sole operative?) or secured.

Forklifts with sufficient capacity are rare beyond dockyards. PLS would be ok if the contents were secured properly i guess but messy otherwise
Best
Mike