Regarding the author's rōmaji spelling of Marin's name, I'm pretty sure that the E at the end is there because it's meant to be the English word "marine" rather than to imply anything like a final ネ. Like how "tone" would be transcribed as トーン, Hepburn tōn, but more typically given as "tone" due to that being the spelling of the word it's based on.
So rather than a mistaken spelling, I think it's probably just the author not realizing that English speakers usually write the Japanese name マリン as "Marin" rather than "Marine," even when it is a pun on the word "Marine."