Yes, frozen grapes are literally made by just sticking a bunch of grapes in the freezer. I've only ever done it with green grapes, but I imagine it would work with red grapes too. A friend in Australia introduced me to the idea ages ago when we were teenagers, and I had a similar reaction to you: I couldn't believe I'd never come up with such a clever idea myself. The grapes have a consistency somewhere between grape sorbet, and grape-flavoured ice, and are deliciously cooling.

I wouldn't really say Thorn reinvents the fairytale — it stick pretty close to the arc of the original sotry. What it does is flesh that story out — fairytale characters don't tend to have any emotional/psychological reasons for doing the things they do, they just do them because that's what the story requires (or because they are generically 'good' or 'bad') — but Thorn ascribes motives and personalities which explain the choices characters make. And so the heroine is happy to make a political marriage to a stranger because her family are abusive and she sees the marriage as a way to escape the abuse, the servant (or rather aristocratic lady in waiting) who tricks everyone into thinking she is really the princess does so because the princess ruined her life several years back, and so on. There's also an added strand about the impossibility of the poor and dispossessed residents of the kingdom getting any form of justice from the authorities, and turning to their own networks of thieves and criminals to dispense justice instead (and generally about the nobility/royalty being so removed from the people they rule that they are incapable of addressing their concernes), and another strand about intergenerational trauma, and the degree to which people are responsible for the injustices perpetuated by their ancestors. But I wouldn't say that any of that reinvents the fairytale — just bulks it out.