Sawyer Faye is spunky and fun. :) I think it's a really unique combination of tomboyish and contemporary mixed with vintage and feminine. (And I like that both Rowan and her sister would have single-syllable middle names with an A sound...just a cute little connection)
I also want to suggest Barrett, which rhymes with Merit but has a more contemporary feel to me that Rowan and your other names have. It is also unisex. Strikes me as a name that would appeal to you.
You may also like:
Quentin (on a girl? why not!)
(you into word names?...)
First of all, I love seeing Alys on your list! It's so beautiful and rare. I really like Asher Alys! It's absolutely adorable.
Have you considered spelling Phelan as Fillan/Filan or Faelan? All legitimate spellings. Faelan Alys would be to die for. I also very much like Morgan Alys; it feels so medieval.
Have you considered...
Alice and once I saw the Welsh Alys, it seemed perfect (my grandmother is Welsh, moved to the US when she was young)
I do like the spelling Faelan but I worry people will assume it's made up or overly trendy.
I like Lavin, Marin, Rosai, Dillian (and Dylan as well) from your suggestions. It seems like I'm always picking names that are either skyrocketing in popularity or already in the Top 100. Rowan's name is only in the mid 300's and I hear her name waaaaay too much, I can't imagine if I used Morgan or Dylan... I would hear it all the time (granted, mostly on boys.) but STILL. :(
*I wanted to add my reasons for loving this name. My dad who passed away loved boats and the water, and he also had an addiction problem. There is a Grateful Dead song called "Lost Sailor" which makes me think of him and the battles he faced.