Agh. This is a mess, although fortunately much less of a mess than many others: I think that either option will turn out ok in the long run.
First, an open question. I think Hannah is entitled to use her own judgement about what Jack would have been ok with. If she thinks Jack would have said “Oh dear. I wanted to be cremated, but I honestly don’t think it matters, so since I’m dead anyway, I’d much rather not cause unnescessary pain to my mother. I’d like to be interred,” then she can do that. And, more likely, Jack might be happy with a compromise, like being cremated, but the ashes stored with his mother’s plot.
Secondly, the usual disclaimers. If someone actually said what Grace said in real life, I’d infer she’s having an understandably very emotional experience, and perhaps doesn’t literally mean what she says: for instance, if talks it over with Hannah, perhaps she’d be equally happy with the ashes being in the tomb, or having a funeral service later after the ashes are scattered, etc. So I’d suggest talking it over. And the entire problem might be solved. But I infer from the question that she is supposed to have TRIED to come to terms with it, over a period of time, but still can’t.
Thirdly, the reason why Hannah should honour her implicit promises to Jack. Obviously it’s not at all obvious why she needs to, but conversely, it is part of society that we DO try to do so, even when promises COULD be broken with expediency (because the principle isn’t there or is dead). I think it’s disingenious to just say “because I can’t see the downside, I should ignore it and it’s stupid to do otherwise”. Very probably that will be the conclusion, but I think that people WANT to keep promises suggests there is some reason, even if an obscure one. I think it’s like the prisoner’s dilemma: even if in each individual case doing the expedient thing is lots better, if EVERYONE does it, people won’t be able to promise at all, because no-one will believe them, and then you’ll waste lots of resources on people trying to legally enforce their will in various circumstances. If so, it’s ok to BEND the promise, so long as everyone tries to gloss over that fact as much as possible. (For instance, Hannah exaggerating to herself that Jack would want to keep his mother happy.) I don’t normally like self-deception, but in this case I think it’s healthy.
In conclusion, I don’t know. I’d look for the best compromise I could and hope for the best.