No, Nimloth was not Elwing's reason for choosing an elven fate. In the published Silmarillion, she was said to do so because of Lúthien. Presumably, as a 'counter' to Lúthien's choice to be mortal. Nor did she overrule Eärendil - he asked her to choose, and he went with what she said so they would remain together. We are told it was not his inclination, but he specifically asked her to choose for them, without stating his own preference. Regardless, Nimloth will (eventually) be Dior's wife and Elwing's mother in Silm Film.
In our adaptation, many of Elwë's kin are killed by the Hunter at Cuiviénen. Celeborn survives, and is established as Elwë's kin, but the precise relationship is never stated. I recognize that Tolkien came up with a family tree, but we agreed to never name Elmo on screen in this adaptation, so it's rather a moot point. We've also made Celeborn significantly older, as an elf from Cuiviénen, so that family tree is not accurate for our show. Galathil, in our adaptation, is Celeborn's sister who lives among the Nandor (and has since they became the Nandor). She will survive into the Third Age as well. She was brought into the story for her reaction to the Galadriel and Celeborn relationship. Since they went to live in Ossiriand, not Doriath, after their wedding, it's clear how that worked out.
We have not yet introduced Galathil's husband, nor have we established that she has a daughter. So, as such, Nimloth is not yet a character in Silm Film, and when and how she will be introduced has not yet been determined. So, there is still time to discuss her origin. I see no reason to introduce her in Season 6. While she may play a significant role in future seasons, we have not yet planned out those plotlines.
As for dynastic marriages, that is decidedly not the elven approach to these things, and first cousins is of course not acceptable. Elrond is also, though, related to all of the characters in the Silmarillion in some way.