There isn't a carb limit in the book. It is more that if you avoid the specific foods that aren't allowed and stick to 800 cals, you will naturally achieve a low enough carb count. The plan is about regulating insulin and doing so by eating foods that don't spike your blood sugars.
The menu plans are good, but if you follow your own recipes you must really use those plans as a guide- eg. If he uses porridge oats on one day a fortnight, that does not mean oats are 'allowed' - it means one meal out of 42 should contain oats. I see lots and lots of incorrect advice, particularly on the Facebook groups, where people are making oat cookies and having them every day, or having them every day. That is not the plan.
You are best to avoid pasta, rice, grains, potatoes and sugar pretty much constantly. He does use occasional quinoa and wild rice, but again, it's one recipe out of a whole week or fortnight. If you do this, and stick to 800 (or even 1000) - plus avoid fruit except berries/apples/pears etc as mentioned in the book, you would find it pretty hard to exceed 50-70g carbs.
I suppose it depends on what works for you. I know I can exceed the calories, but that I have to be very mindful of the carbs, especially those in dairy. I know that for me, fruit is pretty much off limits, especially apples and pears as they make me especially carb-crazy.
I have forgotten the name of the poster who asked, but yes. You really should read the book, even if just as a motivator.
With regards ketosis, I don't know much about it but I know from my gestational diabetes days that it carries some unpleasant side effects and isn't necessarily the best option for everyone.