das Verständnis jener Menschen, mit denen sie leben - Why "jener"?

Question

Previously I thought that masculine genetive of "jeder" is "jedes" (which is also shown here), and after some searching about "jener" on Duden, there isn't any information about the usage of "jener" in the genitive case.

So why not:

das Verständnis jedes Menschen, mit denen sie leben

Source text:

Überlebende von Unfällen und Katastrophen haben häufig Probleme bei der Verarbeitung ihrer traumatischen Erlebnisse. Die Betroffenen leiden unter posttraumatischem Stress, Schlaflosigkeit, schlimmen Albträumen und Panikanfällen, die in direktem Zusammenhang mit den erlebten Situationen stehen. Diese Menschen brauchen unbedingt Hilfe von Experten und das Verständnis jener Menschen, mit denen sie leben.

Answer 1

jener is not jeder, but instead a separate word (a so-called 'der word') that means 'those'. It is the counterpart to dieser.

Answer 2

You are mixing up two different words:

• jeder (masc.), jede (fem.), jedes (neut.), (no plural)
  (3^rd letter is a d)
  This is an indefinite pronoun and it means: each, every, any, ...
  It can only refer to something that is in singular (there is no plural form of this word).
  This word refers either to a single person or thing together with all other in the same group (»Jeder kannte die Queen«) or separately to every individual person or thing of a group (»Jeder war beim Begräbnis, nur du nicht.«).
  • jeder on German Wiktionary
  • jeder on English Wiktionary
  • Translations from Germ. to Eng. on dict.leo.org: jeder, jede, jedes
• jener (masc.), jene (fem.), jenes (neut.), jene (plural)
  (3^rd letter is an n)
  This is an demonstrative pronoun and it means: that, those
  This word refers either to something that is in some spatial distance to the speaker (»Dieses Buch gehört mir, und jenes dort drüben auch.«) or to something that already has been mentioned before or can be assumed to be known (»Die Summen jener Zahlen sind durch 3 teilbar.«).
  • jener on German Wiktionary
  • jener on English Wiktionary
  • Translations from Germ. to Eng. on dict.leo.org: jener, jene, jenes

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Maybe it also helps when the two sentences are translated into English. (There might be English translations that are stylistic more advanced, but I've chosen translations that use patterns that are similar to the German sentences.)

• Original version

  Diese Menschen brauchen [...] das Verständnis jener Menschen, mit denen sie leben.
  These people need [...] the understanding of those people with whom they live.

• Suggested version (corrected)

  Diese Menschen brauchen [...] das Verständnis jedes Menschen, mit dem sie leben.
  These people need [...] the understanding of every person with whom they live.

Note, that in the original version the noun Menschen is in plural, so the relative pronoun in the relative clause after that noun is also in plural. But the suggested infinite pronoun jede_ can't exist in a plural form, so as soon as you use it, also the noun Menschen will turn into singular and therefor also the relative pronoun in the relative clause, that refers to this noun, must be singular.

• jener, jenes

  • plural

    das Verständnis jener Menschen, mit denen sie leben.
    the understanding of those people with whom they live.

  • singular

    das Verständnis jenes Menschen, mit dem sie leben.
    the understanding of that person with whom they live.

• jedes

  • (no plural form)
  • singular

    das Verständnis jedes Menschen, mit dem sie leben.
    the understanding of every person with whom they live.

Answer 3

In this case you can replace “jener” with “der”.

It is plural masculine genitive (die Menschen, mit denen).

“jenes” also exists (e.g. Der Halter jenes Autos), but is singular.

Answer 4

Jener is no declensed form of jeder, but a word of its own. As elsewhere mentioned it was much more frequently used some time ago as counterpart of dieser, as in

Dieser Apfel ist rot, jener ist grün. (This is also shows that jeder is no synonym for jener).

Since the sentence starts with diese Menschen, mentioning a different group is the standard use case for jener. Note, that jedes is a singular-only word, even if jeder Mensch actually covers all humans. But this is a special case not shared by by jener, see declension table.

While you can replace jener Menschen by jedes Menschen (note, that here also a switch from plural to singular is made), this adds an (in my opinion: inappropriate) emphasis; in your example in the sense, that there is not a single human, whose support is not needed. So for a more contemporary style I would replace it simply by der Menschen (also plural).

For declension issues, wiktionary is a good first choice to look up.