For Trevelyan surnames etc: From a war table letter: Lady Osher Lotharn Trevelyan Bayart of Ostwick.
I got the idea from that, they just tacked each extra name added to the family onto the end, including husband's. So she would have been born a Trevelyan in that case and married a Bayart, so keeping both names. Lotharn could have been mother's maiden name, or grand-father's (mother's side) surname. Ancestry and family trees has been a hobby for mine but I tried to work with what I'd already been given as a reference.
Also we have : Philliam Bernard Aloicious Trevelyan.
Would make my character Kyrrah Trevelyan Rutherford if going off those sole two references.
However with Trevelyan being the youngest child (from the codex for the human Inquisitor) or a mage possibly, plus player preferred stuff, it doesn't matter in the slightest in this regard. Just hadn't seen anyone bring up Trevelyan's current relatives that have a long string of names.
I did take a look at the known Trevelyans we meet through the war table missions for reference, and they seem to basically follow the hispanic/iberic convention of keeping both your family names and just tacking new ones at the end, as you say. Can't really be established that the clan name travels down only through one specific gender, fortunately, as that allows room for each player's personal preferences.
I already had a pre-release headcanon for why my Quizz doesn't carry her father's name, though (her Trevelyan comes from her mother, mom is "Bann Trevelyan" - Ari's father was a Templar and surrendered his surname to his vows long before he met mom), and I knew by that time the weight the clan name carried to Ari's personal identity, so in the end that didn't matter much to my decision, in any case - there was a precedent right one step up the family tree. The best she could do if she were to take Cullen's name would be renaming to "Rutherford-Trevelyan," with the clan still taking priority over the individual, but the other factors (like the sense of identity and the need for object permanence, after Trespasser) weighted more to her remaining just simply "Ariana Trevelyan" in the end.
(The noodlets will definitely carry both names, though, carrying the apparent clan convention forward; just haven't decided the order yet. Will they carry forward as Rutherfords or Trevelyans in the end, that is the question...)