Actually, that's not quite correct. The Dunedain were anyone of Numenorean descent - only the Royal lines of Numenor, Gondor and Arnor were descended from Huor. The other Numenoreans were those of the Edain who left Middle-earth for Numenor, and they weren't all related to Elros.

Ah yes that's certainly true, not all Dunedain are descended from Huor, only those with ties to the royal family (although after so many generations, and so small a population, there's a strong chance that a good many of the Dunedain during the third age do have ties to the royal family).

Can't be that many, otherwise every other Tom, dick and Harry would be going to the Steward of Gondor demanding the crown, or even the Stewards themselves siezing it (unless I mis-remember, its specifically stated that the Stewards were among the purest bloods of the Dunedain but had no relation whatsoever to the Royal line)

I don't think that would be too big of a problem, because Aragorn has a legitimate claim to the throne based on his status as the chieftan of the Dunedain (and oldest son of the preceding chieftan). The "Royal Line" is counted only by either the king's oldest son or the king's oldest child, depending on what time period you're looking at (the laws of royal succession in Numenor change here and there). However, many of the royal families consisted of several more children than just the heir to the throne, who went on to have families of their own, meaning that it's not at all unlikely for other Dunedain to have a blood relation to the Numenorean royalty.

Except, it's via one of the offshoot lines that Aragorn is able to claim High Kingship. He is a direct descendant of Elendil who was a direct descendant of one of those 'other children' of one of the Numenorean Kings. Until Aragorn showed up, no-one in Gondor knew about his claim. Unril then, there would have been nothing to stop someone else of Royal heritage stepping up to claim the crown - in fact as I recall, it happened once before; one of the Gondorian Kings died childless and the King of Arnor at the time put in a claim for the throne, but was passed over in favour of a Gondorean general of Royal descent, but not of the immediate Royal line. Although the 'Royal Line' follows down from Ruler to eldest son/child, if the line dies out, it back tracks and goes down the next available line. If that dies out, it back tracks again and so on.

Okay I have time now so I'll add you to the group Hirumith