[Coldsaturn]

Your way of reading it is what I also want to believe in desperately. Them finally having one myth where things end on a hopeful question mark rather than tragedy and death. But it's infold and I don't trust them, so I always have that alarm ringing off in my ears.

"Or was it literally his actual beating heart, now outside his body?"
in Romirro he says that the specific scale he made into a necklace is /part/ of his heart, whether it's metaphorical, magically linked, or literal.
I remember reading a meta post on twitter where it was pointed out that those blue fish are his scales, so every time he conjures up one he's technically giving MC one of his scales. 
- "the shell-scale cracks and starts to fall apart as she wears it."
Yes because it mirrors the state of Raf's heart and his terminal condition.

- "it was never a scale, just a fish in a bowl that she kept near."
That's the thing though, it was never just a fish, because we see at the end of the story that it turns into a scale, its original form. He does the same in Forgotten Sea when he tests her to see if she will run away from him, he turns one of his scales into a fish that then guides her around Whalefall City before going back to Raf and turning back into a scale. In Sea of Golden Sand he gave the fish/scale to her when he was first gifted to her (and she freed him), and it also has the added implication that he tamed himself for her, since removing a Lemurian's scale allows for people to tame them.

- "the fish (vow and heart) were fine."
I took it as the scale being related to the state of Raf's heart more than anything else, just like in Romirro MC's necklace only responds to Raf's deteriorating state.

- "but now they bound by neither vow nor given heart"
That's where I struggle though: yes we can assume he was successful in breaking their bond (he wasn't actually, it reformed again shortly after), but did she ever give back her heart? I think it was still in her, which is why when she reunites with Raf he tells her "I may actually take your heart". In Romirro she doesn't just command him to take back her heart, but also to end their bond, because those are two different things and you can have one without the other.

I really really really want for Abysswalker to have an open ending or even a happy ending like your interpretation highlights, I'm just too cynical by now to believe infold would actually go in that direction lol
But I would love for you to be right so much.