I have zero personal experience of this but a couple of years back I listened to the podcast The Great British Cult and I always thought I would go back to the last couple of episodes of that if one of my kids ever got drawn into a similar belief system. Any sort of criticism will work against you and your relationship so it is best to avoid that for now. As said above - being completely supportive/loving but also saying that your job is a parent is to help take the long view.
There are probably too many links to read by tomorrow and you are overwhelmed so it is probably worth taking a step back?
So if it were me I would tackle the autism, the difficult first year and thinking about those things/the bigger picture (DS is autistic so I know a lot about that). Stick to those topics and finding out why she wants to make a change?
If she is unhappy, what are the other possible changes that could be made? Make a list of the things that are difficult and try to drill into those things? For a school age DS I used a map of the school to colour in red/amber/green and we went through a typical day and all the teachers/lessons to gather info because he couldn't say what the problem was. It turns up loads of useful stuff you would never get otherwise. DM me if you want the full instructions.
Maybe she would want to speak to a counsellor rather than you to help get some perspective? Ideally if you can get her a counsellor - @TwoLoonsAndASprout will be able to suggest someone who can help navigate with a neutral stance.
See if you can agree to get a watchful waiting approach and then agree to discuss again in a few days/a week.
Also agreement to avoid a DIY medication route because it is not safe - and if she's going to do it it must be completely safe and properly supervised - if you have a family history of osteoporosis that would be a good thing to mention to encourage caution.
Good luck xx