I think losing weight is just a small part of the bigger picture. Zoe Harcombe got into the nutritional debate for weight loss reasons but she is part of a growing movement of people that are trying to highlight the fact that diseases of the modern world all have their roots in our radical change in diet since modernization.
Everyone thin or fat, young or old need to look at how much processed food and sugar we eat these days and how it is the underlying cause for the increase in diabetes, Alzheimers, heart disease, asthma, cancer, Type 2 diabetes, osteoporosis, stroke....the list goes on. Just because you might be slim you can still be effected by all these conditions. The amount of sugar our children eat now a days for example at children's parties I feel is madness (I'm not advocating banning birthday cakes just the 30 other foods served - even the sausages will more than likely have sugar in them). Just because your child might be slim doesn't mean you are not harming their health by giving them sugar.
I really like the science bits I'm afraid and I think the more you read and look into it the more simple and obvious it all becomes.
I would agree with you that in Zoe's lecture because she's a Maths grad she like numbers and does throw a lot of figures at you especially at the start (it's the same in the book) which could put some people off but I don't think there is much high level science in it that the general public couldn't handle.
In defense of her diet forum I feel it's supplies a supportive encouraging environment for weight loss which without local clubs (although a few have started up) is really valuable to people. You are also preaching to the converted there as most people on it agree with her argument. You are right here mumsnet's remit is much wider than a self help forum and at times because of this can become quite combative so I'm glad the THD forum is not like that.
I also wouldn't want it to look like she's a lone voice out there full of conspiracy theories she is not the only one. I am very much up for voicing debate and dissent but would urge people to read, listen and research as much as possible. As sometimes happens the ones that shout the loudest aren't always the best informed.
Some books and links I can really recommend:
The Diet Delusion by Gary Taubes
Why We Get Fat: And What to Do about It by Gary Taubes
Fat and Cholesterol are Good for You by Uffe Ravnskov
this a summary of his bigger book www.spacedoc.com/saturated_fat_is_good_for_you_1
Nutrition and Physical Degeneration by Weston A. Price journeytoforever.org/farm_library/price/pricetoc.html
Nourishing Traditions by Sally Fallon
an article by an American Cardiologist
lewrockwell.com/miller/miller38.1.html
Real food by Nina Planck
Not On the Label: What Really Goes into the Food on Your Plate by Felicity Lawrence
Eat Your Heart Out: by Felicity Lawrence