The last of the Elven lords who left Middle-earth was not Círdan, as many supposed. Nor was Elladan's departure, leaving his brother Elrohir at Imladris, definitive. Because Maglor, who had survived the wars of the First Age and wandered along the mortal shores for many years at the beginnings of the Second Age, sought at last for a little rest for his spirit with his friend Elrond.

Young Elves didn't know his face, and even those few who had known him in Valinor or Beleriand and were still alive didn't realise his coming to Mithlond. But Elrond, adopted by Maglor after the Third Kinslaying, was able to recognise him.

In Lindon, Maglor stayed even after Círdan's departure, to the North, at the stronghold of Ost Helevorn, where his brother Caranthir had lived long time ago.

But the Elves of Aman knew of the new peaceful era on Middle-earth, and longed for it and at last they joined under the leadership of Inglaur Ingwiel, Ingwe's firstborn and Prince of the Vanyar, and of his cousin Indis, still held as Queen of the Noldor. They asked for Manwë's blessing and permission to come to Middle-earth, and permission was given. So in the year IV 428 they came to the shores of Arnor before the Straight Path was definitively and forever closed, and the King received them.

Thus begun the last trial of Elves and Men to live together, for the so-called Eldennori, the "Eldar of Middle-earth", worked for the recuperation of the lands formerly inhabited by Elves. Thus the effective dominion of the Reunited Kingdom came to Rhovanion and the borders of Forodwaith.

Maglor was recognised by Indis and was named Lord of Lindon by the will of Indis and the King.
 

Until this moment this story is, I think, perfectly compatible with Martin's chronology of "The New Middle-earth Timescape". From here on disagreement begins to creep in, but this can be turned in a very rich "unified" story. I think the fair result of mixing our different opinions (Chris', Martin's, mine) is a combination of diverse "stages" in the government of the Reunited Kingdom and a history that is more realistic than that invented by me alone. This is how the history continued in "my" 4th Age ...

 
In the meantime, the Reunited Kingdom had strengthened its relationships with Khand. In this empire enemy of Sauron, shaped during the Third Age, the figure of the Emperor had lost his power because of Sauron's machinations, especially the work of Úvatha, the ninth Nazgûl. The Variags, north-western habitants of Khand, became the most powerful people of that region and served Sauron without shame.

The Avari lived with independence at the North. But Emperor Burbeld, after the Variags had been called by Sauron, saw that his opportunity had come, and with the aid of the Avari assailed the main cities and towns of the Empire and recovered the power de facto for the Emperors (III 3015-3017).

Sauron's defeat did nothing but to affirm Burbeld's authority, and the people of Khand, even amazed Variags, celebrated the power of the winner. Emperor Dheigas, Burbeld's grandson, and king Esretaf of Amomag (the only Haradan kingdom opposed to Sauron) signed the Frontier Agreement (IV 49) with Elessar.

In the first years of the Fourth Age, Avari began mixing their blood with men of Khand, and later and more frequently with men of Gondor. This was the main link between the Reunited Kingdom and Khand, who thereafter and until their utter destruction were allies.

Moreover, a new people appeared in the north of Khand, mixture of men of Gondor, Rohan and Khand: the Dúnkhands, or Western Khandians. Those links were strengthened by the coming of the Eldennori, but of these few took Men as their husbands or spouses.

But the Household of the Telcontari was ever ennobled by an Elven spouse for the King and their lives were lengthened, until Elessar II married Léofhild (1354-1426), heir of the Kingdom of Rohan.
 

From now on the history is an extrapolation of these initial centuries of the Reunited Kingdom until the destruction that put an end to the Fourth Age. This destruction playing the role claimed by Bernie Roessler in the last lines of A World Turned on its Side (Reunion 5).

 
One only of Morgoth's great servants remained. This was Umuiyan, who had dwelt since the destruction of Angband in hidden caves of the northern Forodwaith, in the rests of the destroyed and drowned Utumno.

When Sauron was overthrown, Umuiyan reunited the remnant of Morgoth's vassals, because those among them who were Sauron's rivals had been waiting. But Umuiyan was not known to the Elves, Men or even Valar, and he did not appear suddenly, but hardened the climate and supported the enemies of the Reunited Kingdom and Khand.

At last, in a great deed of power, Umuiyan captured Ossë and controlled that powerful spirit and then sent his fury against the shores of Middle-earth, mainly of the Reunited Kingdom.

This unfolding of power awoke the Balrogs hidden under Hithaeglir and Ered Mithrin, who had been Umuiyan's enemies, and the two mountain ranges were thus destroyed. But since the Balrogs feared the power of the Vanyar they made themselves vassals of the Reunited Kingdom.

This way begun the War of Umuiyan (IV 1773-1951), in which, to protect the inner lands from Ossë's attacks, King Elendil II commanded the Balrogs to create a great mountain wall. Thus were the Alps made.

In this war, Ossë spent much power, and was indeed killed in body (1928), after having changed the shape of the north-western shores of Middle-earth; at last the very Umuiyan was killed by Inglaur (1950). But Esgaroth and Dorwinion, at that time enemies of the Reunited Kingdom and Khand, had already destroyed them.

The Elves followed the summons of Inglaur and retreated to Forodwaith to leave Men and fulfil Ilúvatar's designs.
 

All this is of my invention. Here is the big thing that Bernie Roessler writes about. Making a self-criticism, I have to say that some features are somewhat unbelievable, and that there are some weak points: for example the excessive homogeneity of the history, or why should Elves have left Men? But I think that these defects are solved by the combination of our conceptions. So let's make the mixture ...

 
King Isildur II created the Lordship of Western Gondor (IV 506) for Auredhir, Lord of Pinnath Gelin, who had reunited the lands of Gondor from Lamedon to the Gwathló under his rule. Already powerful, those Lords soon became the most important in Gondor after the King himself, and when Eldarion II sat in the throne (700), he gave Galadhroth III of Western Gondor the new title of Attaran, or Viceroy.

But that was a great error. It is a feature of human nature that when an Empire is grown, its provinces want themselves to be great kingdoms. So it happened that Viceroy Hirluin, son of Galadhroth III, grew great in pride and, having bound the movements of the Princes of Emyn Arnen and Dol Amroth thanks to his economical power, named a new King, coeval to the legitimate one, but living at Minas Tirith. For Eldarion II passed much time at Annúminas in the north.

The Elves' hands were bound by the will of Manwë, who had allowed them to depart only under Oath that they should keep their powers only for their houses and lands. Furthermore, Rohan and Khand considered this an internal affair of the Reunited Kingdom, so Eldarion was forced to accept the duarchy.

The southern Kings allowed Hirluin to keep a certain innocence in their acts, but they were completely subordinated to him, and later to his son Helinon, and because they were aged they died before they could strengthen their position.

But about the year 970 Helinon revealed his intention to sit in the thrones of both the South and the North, and in 976 he organised the Southron invasion of Eriador. The next winter Arbelleth, thirteenth of the southern kings, died, and Helinon took the name of Arbalad and named himself King of Gondor and Arnor.

Now were events stronger than the will of Inglaur and Indis, because they loved the country that had received them, and they decided they must break their Oath. So an army of the powerful Vanyar came from the North, and an Eldarin navy debarked at Edhellond.

None offered resistance, though some saw this action as a coup d'état; but the Princes of Dol Amroth and Emyn Arnen rebelled against Arbalad. So he and the people who supported him, mainly Dunlendings but also many other men from Enedwaith to the Pinnath Gelin, fled with him to Harad and founded a little kingdom far inland, out of the influence of the Reunited Kingdom until Umuiyan contacted them through Umbar and the Haradrim. Arbalad never accepted the true situation of the Reunited Kingdom and held his position as a retirement for a future glorious coming. But Western Gondor was given to the house of the Princes of Tharagrond, who worked for restoring the normality and even took names following the tradition of the house of Pinnath Gelin.
 

Well, that's one possible solution. I think it serves Martin's purposes, (though I don't know too much about the Book of Visions), and mine, and no doubt it's much more realistic and rich in its variety. For example, the division of the Reunited Kingdom because of a state of "unrest" recalls too closely, in my opinion, the division of Arnor, though the North-South division would be a tendency marked by geography.

Having said that, the state of unstable equilibrium that I describe above doesn't appear in the "legendary history" of north-western Middle-earth (Castamir's and Wulf's are utter usurpations), and it is somewhat based on the confused situation of the Roman Empire in the third century.

Arbalad's people would have kept their version of the difficulties on the Reunited Kingdom, and they would have survived the War. This War of Umuiyan and the definitive severance of Elves and Men can thus be explained as a punishment for the breach of the oath of the Eldennori (that is substantially different and opposite to Fëanor's).