How can the details of the mappō age be discussed skillfully?

The destruction of society represents the turmoil of the samsaric cycle. I think it can be helpful, not unhelpful, for practitioners to see the unwholesomeness in the world or, to use your words, the grim picture. We come to understand maturity and immaturity and this sets a marker from which one can study the behaviour of humankind and how that behaviour influences our thoughts, speech and actions. After some time, we discover that we no longer want to look at the world in such ways, we discover that we don't like what we see in the world, and we may feel bad towards them - but this is helpful. It is a kind of strange grief that, if embraced, leads to a great opening imbued with great wisdom alongside the unmanifested.

Now, here's the thing: With all the world's stories put on pause, what is actually happening within the various dualities of the world is an affectionate gyration of love in all its many guises, and that those seeming individual manifestations (the ten thousand things) are the innocence of that love.

The optimal way to address, and to discuss it skilfully, is to recognize this love in the way the Buddha phrases in the Lotus sutra...

According to how all the living beings

In previous lives put down good roots,

They have knowledge of the mature

And of those who are immature,

They take it all into account,

Articulate it in their understanding,

And following the way of the one vehicle

They appropriately expound the three.’

We are in the Latter Day of the law. There are three periods of time that the Buddha said would take place after his death. The first period is named the Former Day of the Law. During this time, many people attained the way and were able to adhere to the precepts. During the middle Day of the Law, Buddhism began to deteriorate and not many people attained Buddhahood as the amount of people who did in the Former Day of the Law. During the Latter Day of the Law, almost everyone is tainted by greed anger and Foolishness. That is not a joke. This is a scary time we are in. Almost, zero people will gain enlightenment. Just look at the world today. The Buddha saw all of this coming. If the Buddha told his disciples about the Latter Day of the Law, then ordinary teachers of the present time should do the same. Here is a Sutra to support what I have said.

The Nirvana Sutra says: “There are icchantikas, or persons of incorrigible disbelief. They pretend to be arhats, living in deserted places and speaking slanderously of the correct and equal sutras of the great vehicle. When ordinary people see them, they all suppose that they are true arhats and speak of them as great bodhisattvas.” It also says: “After the Former Day of the Law has ended and the Middle Day of the Law has begun, there will be monks who will give the appearance of abiding by the rules of monastic discipline. But they will scarcely ever read or recite the sutras, and instead will crave all kinds of food and drink to nourish their bodies. . . . Though they wear the clothes of a monk, they will go about searching for alms like so many huntsmen who, narrowing their eyes, stalk softly. They will be like a cat on the prowl for mice.” The Parinirvāna Sutra states, “There are also icchantikas who resemble arhats but who commit evil deeds.”

"it gives practitioners (especially beginners) the feeling that the world is falling apart", that's totally straight to point: urgency, since it soon breaks for one appart. If a fool nevertheless does not go after right effort, what else could the great Teacher have left behind for him... so what is good householder waiting for not to seek for going forth, now if aware of the danger around?

Fear of the world (of senses), seeing it's hoples bound to corruption, saṃvega, is the very prequisite for Refuge into the Gems, Awakening, and so most skillful if one is able to ser, confirm the 1. Noble Truth in all aspects: Affirming the Truths of the Heart: The Buddhist Teachings on Samvega & Pasada

See also 'future' dangers, and more detail, urgency reasons.

The purpose of the teaching of the degenerate age of Buddhism, is to remind monks and students of the Dhamma that they should be cautious not to be enchanted by the elegant words of contemporary unenlightened self-proclaimed gurus, teachers and prophets, while neglecting the true Dhamma, the teachings of the Buddha.

Instead they should reflect: 'We will listen when discourses that are words of the Tathagata — deep, deep in their meaning, transcendent, connected with emptiness — are being recited. We will lend ear, will set our hearts on knowing them, will regard these teachings as worth grasping & mastering.'

"Monks, there once was a time when the Dasarahas had a large drum called 'Summoner.' Whenever Summoner was split, the Dasarahas inserted another peg in it, until the time came when Summoner's original wooden body had disappeared and only a conglomeration of pegs remained.

"In the same way, in the course of the future there will be monks who won't listen when discourses that are words of the Tathagata — deep, deep in their meaning, transcendent, connected with emptiness — are being recited. They won't lend ear, won't set their hearts on knowing them, won't regard these teachings as worth grasping or mastering. But they will listen when discourses that are literary works — the works of poets, elegant in sound, elegant in rhetoric, the work of outsiders, words of disciples — are recited. They will lend ear and set their hearts on knowing them. They will regard these teachings as worth grasping & mastering.

"In this way the disappearance of the discourses that are words of the Tathagata — deep, deep in their meaning, transcendent, connected with emptiness — will come about.

"Thus you should train yourselves: 'We will listen when discourses that are words of the Tathagata — deep, deep in their meaning, transcendent, connected with emptiness — are being recited. We will lend ear, will set our hearts on knowing them, will regard these teachings as worth grasping & mastering.' That's how you should train yourselves."

SN 20.7

Tough times are inevitable:

DN26:19.1: There will come a time, mendicants, when these people will have children who live for ten years.
DN26:21.2: During that time they will see each other as beasts.
DN26:21.3: Sharp swords will appear in their hands,
DN26:21.4: with which they’ll take each other’s life, crying, ‘It’s a beast! It’s a beast!’

Yet for some, just as inexorably, prudence and conscience will provide an escape from tough times:

DN26:21.5: But then some of those beings will think,
DN26:21.6: ‘Let us neither be perpetrators nor victims! Why don’t we hide in thick grass, thick jungle, thick trees, inaccessible riverlands, or rugged mountains and survive on forest roots and fruits?’

From that narrow escape will spring hope, faith and skill:

DN26:21.10.0: 6. The Period of Growth
DN26:21.11: Then those beings will think,
DN26:21.12: ‘It’s because we undertook unskillful things that we suffered such an extensive loss of our relatives.
DN26:21.13: We’d better do what’s skillful.

Today we see signs that remind us of that very perilous decline. Seeing those signs, should we give up and roll down the cliff?

Or should we attend to what we can skillfully do?

DN26:22.1: Then those beings will think,
DN26:22.2: ‘Because of undertaking this skillful thing, our lifespan and beauty are growing.
DN26:22.3: Why don’t we do even more skillful things?

Should we read the full sutta or just stop at the horror?