[Rumil]

Hmm, lets see if I can sort this out..

Elendil was father of Isildur and Anarion and therefore the 'High King'. He set up a kingdom in Arnor, while in Gondor, Isildur's capitol was Minas Ithil, Anarion's was Minas Arnor.

During the War of the Last Alliance, Minas Ithil was taken by Sauron's forces, Isildir and his family escaped to Arnor to gain Elendil's help and left his son Valandil at Imladris since he was too young to go to war. Now, Elendil and Anarion were both killed in the war. Isildur gave Gondor into the care of Meneldil, son of Anarion, and set off to Arnor. At the disaster of the Gladden Fields he was ambushed and kiled, so the crown of Arnor came to his young son Valandil. I guess officially he could have been viewed as the 'High King' but by the time he had grown up Meneldil was too firmly established in Gondor, anyway at that time it could have seemed sensible to split the kingdom along the obvious geographical lines.

I think the point is that Elendil was accounted King of both Arnor and Gondor. By the time of Aragorn, (a direct descendent of Isildur) the line of Gondor was no more so Aragorn claimed the throne of both Arnor and Gondor, re-uniting the kingdoms.

There was an earlier attempt to reunite the kingdoms but it had failed due to Gondorian unwillingness (and power politics naturally).

I'm not sure of the relationship of the Stewards of the House of Mardil to royalty, but if there was any it must have been very distant, otherwise they would certainly have claimed the throne of Gondor when the line of kings failed.

Hope that makes some sense Meela!