1) I'd say yes, the story of Mithrellas and Imrazor does appear to show that
there is an elvish ancestry to Dol Amroth. As to comparing their story to
that of Beren and Luthien, don't forget that Luthien is herself only half
Elvish. Her mother was Melian a Maiar, who existed before Arda or even Ea
were even thought of. I think it was this 'angelic' ancestry which
allowed their descendants a measure of choice over which 'race' they could
belong to, Elven or Men, as they had a foothold outside of both. Who
knows, maybe there were other unions of Elves and Men? There didn't seem
to be any insurmountable barrier between the races, even though there were
obvious difficulties. In the debate of Andreth and Finrod we learn that
the wise woman Andreth and Finrod's brother Aegnor were very close, and that it
was only the closeness of War that prevent their union. I'd guess though
that if there were the outcomes would be similar to Mithrellas and Imrazor,
with the Elven partner leaving in the end.
2) As far as Aragorn's not wanting to push things, I'd say he was being very
wise there. The Lord Denethor, who the people had been loyal to for
decades, had only just died, and under obscure circumstances involving his
friend Gandalf. In their grief there could have been some who would have
been suspicious of Aragorn, just as Denethor himself
was.
