[Boromir88]

Originally Posted by Zigûr

The Oath refers particularly to the mutual friendship between the kingdoms against "the Shadow in the East" and "the Shadow" in general; I can imagine in time arguments being raised as the realms became more 'political' that, with the defeat of the Dark Lord, the Oath no longer carried the same weight.

I disagree, I think the oath refers to any threat, not just specifically Sauron and an oath of eternal friendship:

Hear now all peoples who bow not to the Shadow of the East, by the gift of the Lord of the Mundberg we will come to dwell in the land that he names Calenardhon, and therefor I vow my own name and on behalf of the Eotheod of the North that between us and the Great People of the West there shall be friendship for ever: their enemies shall be our enemies, and their need shall be our need, and whatever evil, or threat, or assault may come upon them we will aid them to the utmost end of our strength. This vow shall descend to my heirs, all such as may come after me in our new land, and let them keep it in faith unbroken, lest the Shadow fall upon them and they become accursed.~Unfinished Tales; Cirion and Eorl

"for ever" and "whatever evil, or threat, or assault" I believe means more than their current threats from "the Shadow of the East."

I do agree here:

The idea seems not wholly inconsistent with Professor Tolkien's view of the declining moral fibre of Men as the Fourth Age wore on~Zigur

"Sworn oaths" lose their significance over time. They don't mean anything, because as you expertly put it Tolkien's view about "declining moral fibre."

I just don't see rifts/tensions/war between Rohan or Gondor occurring during Eomer and Eldarion's time, or Eldarion's and Elfwine's. It won't come until the point where Men's moral decline has fallen and sworn oaths mean nothing to Men. Such a time in unforeseeable, at least unforeseeable to happen during Eomer and Eldarion's time.