OK! I slightly understand that. But I do not have root permission for further investigation.

/ m n t / e x t _ s d c a r d work for me as visa versa of / s t o r a g e / 0 0 0 0 - 0 0 0 0 on my phone.

The marine profile is on the path in APK:
res\xml\marine_defaults.xml

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The strings that were highlighted by red is a question. I am going to try to substitute one of a visible profile like Motorcycle with the content of this marine profile in APK and see that will be happened...

/ d a t a / d a t a / c o m . g a r m i n . a n d r o i d . a p p s . o u t d o o r is obviously used as 1:/ (nonvolume) drive?! Can you see if it have *.gtt files?