Haha, always a great ship! I love the matching trope.
I don’t love or hate transliterated accents in written form, but if the accent is too thick (or, of course, on-point in terms of accuracy), it can take away from the reading experience. I remember 2DK but don’t remember exactly how the accent was presented. For this situation, if I read the Scottish accent version first, I probably wouldn’t understand a thing (at least, if I were only reading). But Mary/Marnie’s accent, since it only cuts letters here and there, reads fine.
For a longer, serialized work, I can see why a fully-transliterated accent can make reading difficult and ultimately cause people to drop a series. Juggling authenticity and readability is always tough.
(I’m currently an editor for a romance sim game. There are some medieval-type books that could’ve had accents within the dialogue, but we favored readability. Sucks, but we figure it wouldn’t ostracize readers.)