Alright! I've pared it down to 158 words, minus the vote markers. Hopefully that's more reasonable.
[X] Sober up. This is serious, yet good.
-[X] Step 1: Invite
--[X] Confirm her suspicions
--[X] Offer to explain as best as you can.
--[X] Warn that some topics are sensitive, and why
-[X] Step 2: Inform
--[X] Besides the obvious dangers (fighting witches), being a magical girl has hidden dangers, and we want to dissuade Contracts.
--[X] Segue to Hitomi not being a potential, and explain the first hidden danger: "Grief" is literal.
---[X] Subtly apologize to Mami, and explain that you've been planning to tell adults about magic for many reasons, this foremost.
--[X] Connect the resource-management of Grief to Witches, and the second hidden danger: Scarcity.
--[X] Lead into our powers (and those of the team)
--[X] Walpurgisnacht.
-[X] Step 3: Inspire
--[X] Discuss what we want to accomplish once the immediate problems are resolved.
--[X] Make sure to stay away from the Lichbomb, Witchbomb, etc.
---[X] ...but try and set up an opportunity to talk with her in a context where we can discuss them (and other heavy topics).
Step 1 is all about expectations and perception. We want her to believe us when we tell her some pretty extraordinary things, and we want her to trust both our intent and our ability to keep Hitomi (and others) safe.
To do that, I think that the firt thing we need to do is to come clean, and well. She needs to believe us when we talk in the future, and the best road to that is being as upfront as possible when given the opportunity. However, the various infohazards mean we also need to do our due diligence.
As a result, I think the best option here is to unambiguously say "Yes, that is what's going on", paired with an offer to explain the parts of the iceberg below water and a warning that some topics need handled with extreme care - either because they involve personal information we would need permission to reveal, or information that could put people very directly in danger.
If Step 1 is about expectations and perception, Step 2 is about staying safe and keeping others safe. In order to help with our problems, rather than being ineffective (or making them worse), she needs to understand them.
Explaining "Grief" goes first for several reasons; one of the biggest is "she can actually do something about it" and the second-biggest is "because it's one of the most dangerous" - there are very few ways to solve a grief spiral once it starts, and we can't be everywhere. Making her aware of the issue means that we have help in monitoring such things, and can catch issues much earlier - early enough that nonmagical intervention may suffice.
By a similar token, that also explains the role Hitomi and Madoka play, and why we associate with them - this, then, has strong implications of the lengths we'd go to for their sake, as well.
The problem of Grief then connects very naturally to why we hunt witches, and I don't think we'll need to explain much further than "Witches are a scarce resource" for her to get the (full, ugly) picture. She's a businesswoman, and a sharp one.
However, she does need to understand that as far as scarcity goes, we're a massive game-changer. Quantifying Clear Seeds' cleansing capacity vs. normal Grief Seeds, as we've done before, should give a very solid sense of the scale we operate at.
Of course, discussing our power leads nicely into discussing the powers of the other members of the team; this should help counterbalance the concern she's almost certainly going to be feeling from how we've detailed all of the dangers she didn't know about. Our group is exceedingly well-equipped - our power takes scarcity of Witches out behind the woodshed, Homura's power means we can respond to almost any situation far more effectively than would be even vaguely reasonable, Sayaka's growing collection of power charms gives incredible flexibility, Mumi is Mumi, and we've formed alliances with other groups as well.
But she does need to know about Walpurgisnacht. This is where we lay all our cards on the table: What's coming, what we've done so far, what we plan to do, and what we hope she can help us do. Handling Walpurgisnacht will need plenty of things besides just fighting it - we set up the storm warning to reduce casualties, but being an adult gives her a lot of options we lack. In addition, the influx of meguca to fight Walpurgisnacht is going to need various kinds of prep - food, shelter, etc. Even beyond that, we're not perfect. We might have missed something; being open about that and asking for someone to cover your blind spots can only help.
Once that's done, we need to show her that we're not just trapped in short-term thinking. Talk about our vision for a post-scarcity world for Meguca, and our thoughts on how we can get there. We need to make clear that we don't just want her to follow our plan; we need people who can ask the questions we didn't think to ask, and propose the solutions we didn't realize were there.
Of course, despite our statement of being open, there are things we just plain can't cover with the current group. She'll probably pick up on that. That's why we really, realy ought to set up a second meeting: We are currently a single point of failure along multiple axes, and we should be trying to improve that. The more trustworthy, competent people we can trust to manage the various bombs, the better off we are in averting them. She also needs to understand Kyuubey, its behavior, and how it will likely end up acting counter to several of your goals.
Finally, when preparing for the second meeting, we should have time to look into Tsuchiya Ritsuko properly. This will allow us to be cautious (pumping Kyuubey for information) and diligent (perhaps Nadia doesn't know, but knows who knows) where necessary. In addition, it looks much more human than firing off an urgent telepathic question to Nadia (somewhat rude) and responding immediately with information you wouldn't have been able to obtain by any normal means.