[Mithalwen]

Eomer and Eowyn, and Theodred definitely had Gondorian cousins and, in a rather Hapsburgian fashion married two fo them. Although Morwen is named of "Lossarnach"it says, probably in Unfinished tales since I can't find it in the appendices to LOTR, that she was of the line of Dol Amroth and Imrahil acknowledged the kinship between him and Eomer of blood even before they were linked by marriage. It seems reasonable to suppose that the daughter of a Prince of Dol Amroth married the lord of Lossarnach were the parents/ grandparents of Morwen. The first cousins Faramir and Lothiriel were thereby perhaps third cousins of their sibling spouses. Though the exact degree cannot be certain. Their marriages surely would also have resulted in further intermarriages since both Lothiriel and Eowyn are likely to have taken some of their own people with them to their new lands and the forging of such strong ties and relative peace would have increased traffic between the two realms.

I imagine that there were two types of Gondorian/Rohan intermarriage. I think it says somewhere that the mountain folk of Gondor were stronger in indiginous rather than Numenorean blood. Along the borders of two friendly realms I imagine people intermarried as they willed without thought of technical nationality.

I would be surprised if the union of Morwen and THengel was unique amongst the higher social echelons particularly as the Numenor effect waned and the lifespans and culture became closer. Noble houses tend to intermarry and only a few Gondorian families had very pure blood. However it is possible that while a daughter of Dol Amroth might have married out, the sons might have been encouraged to marry numenoreans.....

It is likely that there were descendents of Eorl living in Gondor but whether they would have been preferred over say the highest ranking Marshal of the Riddermark surviving the conflict I don't know. However, if Eomer's line failed within a couple of generations I would think that Eowyn's desendents might have been acceptable.