As far as the chapter goes, I don't think the tone shifted too much because I didn't notice it until I got to your question.
For the story as a whole, quite well-done though definitely much darker than JoeHundredaire's.
While you've managed to keep the story from hooking my emotions enough to make me stop reading (As much as it's a classic, I couldn't get past chapter 1 of Margaret Atwood's The Handmaid's Tale and I had no trouble reading other dystopian classics like 1984), a view I might summarize as "the evil of magic" is always at the front of my mind.
I suppose the problem is that this story walks that gray area where it's too captivating for me to walk away in disgust but, at the same time, keeps poking at my sensitivity to stories which contain elements I recognize as dystopian but try to cast themselves as something other than dystopias. (It doesn't help that I can't help but feel distaste for stories where characters like Calypso seem to grow accustomed to a Sword of Damocles like your little spell.)
With a little more thought, I managed to tease out how my mind evaluates the former and split it out into a step-wise checklist:
1. Does the story feel less dystopian than reality or equally so? (No. Continue on.)
2. Is it clear from the writing style that these elements are simply the real-world norm for the place and time in which the story was written? For example, the roles of women in sci-fi from the 1940s? (No. Continue on.)
3. Does the author seem to not even realize that these elements are a bad/evil thing? (No. Continue on.)
4. Does the writing of the story give the impression that the characters will grow accustomed to these "plot-relevant evils" rather than triumphing over them or dying in the attempt? (Yes! ERROR! ERROR! Thou shalt not suffer a dystopia to live! Broken/bad/wrong story!)
Or, while I'm not sure it applies perfectly to all situations I've felt this way about, it might be possible to distill it down to one simple rule: "Given a bit of initial benefit of the doubt, does it look like things will end up being more evil in the last scene of the story than the first once the story has had time to get going?"
That sort of winds up being a Sword of Damocles over me as a reader because I have to spend most of the story in a state of indecision over whether I should be empathizing with the characters or not. (If I don't, the story is unsatisfying. If I do but then the ending is "more evil than the beginning", I'll be in a funk for an hour.)
The problem is that we've already got evils on a much more massive scale than in the wizarding world simply through the effects of bad laws, corruption, and human apathy. To paraphrase an excellent short video "The Story of Human Rights" (If you haven't seen it, go to YouTube right now), 27 million people are still enslaved today which is more than twice as many as in 1800... and that doesn't even count the effective slavery imposed by a bad economy and skyrocketing student loan debts immune to bankruptcy in the United States.
...not to mention other issues like how women in Saudi Arabia are effectively considered children in the eyes of the law for their entire lives or how, according to the World Health Organization, 2.5 billion people (over a third of the world's population) lack access to proper sanitation, 780 million lack access to safe drinking water, and 3.4 million people die each year from water-related diseases.
I just can't help but dwell on the evils of creating a setting which takes such a world where Pandora's box has been opened, then adds a subculture which replicates the same problems in a new context, and then uses magic and authorial focus to try to strangle what hope remains. (I have no problem with stories where I can pretend that all bad laws with draconian, irreversible punishments have been stricken from the books)
I also can't help dwelling on how, if they can't safely remove the spell on Calypso, why haven't they looked into "defusing the bomb" by neutralizing the trigger while leaving the spell in place... or Harry accepting being put under an equally irremovable spell which nudges his speech and intent just enough to avoid triggering the spell. I can't help but remember that the only human who will never succumb to a moment of weakness is a dead human.
...and means of enacting laws that work as EFFECTIVE deterrents to anyone ever abusing such spellwork again. (I see Calypso's situation as akin to why, here in Canada, we don't have the death penalty. People who've received a life sentence for capital crimes DO sometimes get found to have been wrongfully convicted... sometimes decades later and it's also a violation of fundamental human rights.)
That aside I do really hope that, when I start looking at the other entries in the series, I'll see more time looking in on "Winnifred and Georgia". Having those two stuck female for a month would be a VERY entertaining comic relief sidestory. (Especially given that, as businessmen, they can't just become hermits for a month to let the author shirk writing the entertaining reactions and interactions with everyone else.)
Oh, and thank you for giving me an opportunity to figure out how to put all this into words. Until now, it was just a mish-mash of vague feelings of dislike and unease that cropped up whenever I read a story with certain traits.
EDIT: And, now that I'm not so fuzzy, I should also applaud you for being the first person I've seen to put proper effort and plausible details into a "ancient families can revoke the Ministry's charter" plot. I've seen it done various times, but never anywhere near this well. (Usually they just use it as an excuse to take the plot somewhere. You write it as something that feels like it simply exists for its own sake.
UPDATE: And thank YOU! While I don't always succeed (especially when I'm sleep-deprived as I have often been lately), I try hard to write reviews like this as often as possible to thank people for giving me free entertainment. Far too often, I don't get any reply at all, let alone one that thanks me.
Having slept on the topic now, I also realized I should have mentioned another effect of that literary Sword of Damocles that this story hangs over my head:
Practically every interaction between Harry and Calypso (but especially so in ones where it would make literary sense like when they're alone together and have let down their emotional armor to be intimate), I start tensing up in anticipation of Harry accidentally triggering the spell so it can be used as another narrative obstacle to slow and deepen the narrative development of Harry and Calypso's relationship. (After all, bumps in the road help to add to the realism as long as they don't destroy the feeling of progress the way a lot of long-running romantic comedies do)
To be honest, It's gotten to the point where, if I were suddenly writing this, I'd do that as soon as possible to kill two birds with one stone. (It'd prevent additional reader stress by ending the anticipation and, at the same time, it'd give Calypso and Harry reason to re-evaluate their attitude toward the spell and to work harder toward finding a solution, which might let you get away with giving the impression that's what you meant all along.)
It wouldn't be perfect since there's so much anticipation already written for people with my mindset to stress over, but it'd be the best fix without a rewrite.
Oh, and I've added this story to my gender-bending fiction index. If you want to take a look and suggest any adjustments to the entry, it's at http://gbindex.ssokolow.com/stories/266
First, thank you for your well thought out and thought provoking review. I don't currently have sufficient net access to give you the response you deserve, but I wanted you to know that I've received your insights, and am giving them a great deal of consideration.
I will give you a more comprehensive reply when I can.
Again, thank you. ^_^