However, if true, why then to people seem to fall off the bandwagon and gain the weight back?
The short answer is that most don't fall off if they make it a lifestyle-change commitment rather than a temporary 'diet' (fad).
What is the rate of weight loss
Rate of weight loss is subjective, each of us is different and there are many variables to consider so there is no specific number, but if you read the success stories on the site (and forum) you will see there are accounts of major weight-loss without hunger. The benefits are not limited to weight-loss though, many see other conditions improve including auto-immune and metabolic diseases. I will emphasize that this is not a nutrition-only program, sleep, exercise and stress-reduction are also key components.
if you have a preferred argument/ link, I'd like to read it.
The best recommendation I can give you is to read the book, though even just browsing the website and forum you will learn a tremendous amount. Use the search tool for specific areas of interest.
I missed seeing anything about ketosis?
Hope you don't mind if I use links rather than clog up the thread with articles...
Sisson has been heavily researching ketosis lately and is about to release a book...
http://www.marksdailyapple.com/get-ready-to-reset-and-go-keto-incredible-pre-order-bonus-offer/This from 2008 !
http://www.marksdailyapple.com/dear-mark-ketosis/His 'definitive' guide
http://www.marksdailyapple.com/the-definitive-guide-to-keto/Is keto for everyone ?
http://www.marksdailyapple.com/is-keto-for-everyone-cautions-caveats-and-contraindications/There are lots more ketosis-related posts...
http://www.marksdailyapple.com/?s=ketosisAre you on the program? Does it work for you?
I have been Primal since around 2010, my first experience involved getting down to under 100g carbs per day (many times lower, but rarely into ketosis-zone). I went through 10 days of Herxheimer, then when I came out the other side my energy levels went through he roof and I had become fat-adapted. I still play soccer competitively (aged 50) against college and high schoolers, rarely eat before a morning game and have no issues running and sprinting for 90mins. I will add one note that when I first changed to primal I did notice an initial drop in energy, presumably because I was still adapted to burning carbs as my primary energy source.
There are definitely days when I allow myself to cheat a little without guilt, though my body will frequently remind me that I'm not eating high quality food ! And the best part of being Primal...fried eggs and bacon...and butter

I also took the liberty of sending Leung's theory to Sisson to see if he had any thoughts. It's a long shot because he receives thousands of emails daily, but I'll let you know if anything comes of it.