This sounds all the more plausible in light of Tolkien's note (in UT):
They were a family of the Faithful who had sailed from Númenor before the Downfall and had settled in the land of Belfalas, between the mouths of Ringló and Gilrain, with a stronghold upon the high promontory of Dol Amroth (named after the last King of Lórien). If Dol Amroth was a pre-Downfall settlement, it could reasonably claim to be 'more Numenorean' than Gondor proper, which was after all settled by Gondorian
refugees. I can certainly imagine a certain amount of isolation - and, heck, why not occasional (during times of weak kings) interbreeding with the
other pre-Downfall Numenorean state, Umbar? They're both maritime nations ("oh, pardon me, Your Majesty, I mean of course 'loyal subordinate princedoms of Gondor'"), so they had a fair amount in common, at least when they weren't actively killing each other.
I vaguely recall a statement that the bodies of mortal women wouldn't actually be able to bear a half-elven child. I... have no idea where that came from, though.
There is, of course, the case of Aegnor and Andreth, where she was the mortal. Aegnor refused to wed her due to there being a war on (even if it was a cold war at that point), but there's no indication that there were more serious problems.
And
that... makes a worrying amount of sense. Mithrellas, along with Aredhel, is I think one of only two wives to actively run away from their husbands in Middle-earth (plus maybe Miriel, if 'I'd rather stay dead/oh, now you're dead I'll go back to living' counts). That says pretty terrible things about Imrazor, and the idea that she was pretty much a captive rings true.
Over in
another thread you made mention of Gilmith, daughter of Imrazor, and that set me to thinking: did the children of Imrazor have the same Choice as Elrond, Elros, Arwen and her siblings? We know that Galador was mortal, but his sister could well not have been.
How is that Choice made, anyway? Indications from the canon seem to be that 'you get what you marry' - Arwen accepted mortality when she married Aragorn, Elros of course married a mortal, while Elrond and his sons apparently put off marrying for quite some time, and Celebrian was an elf. So could Gilmith have simply remained unwed, and attained immortality that way?
(Line of thinking partly inspired by the lovely image used for Gilmith on Tolkien Gateway,
taken from Lady Elleth of deviantArt.)
hS