Hi, Olivier. I can identify one potential obstacle and one workaround.
Contexts are not actually hierarchical, they are a network, as in @a contains @b, @b contains @c, @c contains @a. I question whether this is a particularly useful feature but that's how it works at this point. This kind of thing would be difficult to impossible to portray in the outline format that you are proposing.
For a workaround, consider varying the first character of each context name. For me, a context starting @ is a person, # is an area of focus, ¤ is a place, ! Is a tool and so on. Ease of selection is achieved by typing the first character and receiving a short drop-down list of included contexts.
-Dwight
Mlo betazoid on Android sgn2