JimBullardBullard, I'll try and answer all your questions as well as I can but I'm sure Stuntnun, wombat or BIWI will be able to correct me if I'm wrong on anything!
Why so much fat:
When you remove a large percentage of a macro (protien, carbs or fat), you have to replace it with something otherwise your overall calories will be too low to sustain normal life and you'll get very poorly very quickly or the diet will become unsustainable.
Why fat and not protien then? Out of the 3 macro's that make up your food, fat has the smallest effect on your blood glucose level (blood sugar - BGL). This is important because the feeling of hunger that we experience every day is your body's way to tell you your blood sugar level has dropped and you need to eat something. Interestingly and importantly, it's not the actual level of sugar in your blood that your body recognises, it's the difference between it's highest point and the point it's at now. So, if you constantly keep your blood sugar low, it can't drop very far to trigger hunger. When our BSL reaches a certain level, the body releases insulin to store the excess sugar as fat. Protien raises BSL to a lesser degree than carbs, but can still raise it enough in large quantities to trigger this insulin response. Fat can't reach those levels of blood sugar unless you were eating enormous quantities, so dietary fat doesn't get stored, it gets converted into ketones and used as energy. For this reason, fat makes us feel satisfied faster than carbs. It doesn't make you feel "full", that's a misconception, it dulls your appitite so you eat mucvh less overall.
Finally, your body has to work much harder to turn fat into ketones than it does to turn carbs into sugar/ fat. Your BMR (metabolism) is therefore faster and you burn more calories just by increasing your fat in take.
Also, if you're trying to get into nutritional ketosis you need plenty of fat, that's where your energy is going to come from. It's easier for your body to burn stored fat for energy if it's already in ketosis from burning dietary fat so as long as you're working on a calorie deficit, you'll lose weight.
What happens if you don't get enough fat: Simply put, the immediate effect is you get hungry and bored very quickly. You also need fat. There's a number of essential fats that you need for life, there's no such thing as an essential carb.
Water: When your fat cells release stored fat, they grab hold and retain water much much more easily. If you don't drink plenty, they keep hold of that water for longer which means you don't then show a weight loss on the scales. You'll still have lost fat but it can be very demotivating to see the scales stay the same or even increase from water retention. The more you drink, the less you retain.
Headaches: How much are you drinking? If over 2.5 litres, you're probably lacking salt. Drink a stock cube dissolved in water. It could be keto flu, it would be delayed if you've been eating over 50g carbs a day and not eating enough fat because you wont have been sufficiently fat-rich to get into ketosis qnd probably still functioning in glycolysis by just about maintaining your glycogen stores. If your glycogen stores are now empty, you'll be transitioning into ketosis which can be rough for a few days.