In order for your organs to live and function for 24 hours, you need at least 1200 calories. For someone who is sedentary and extremely short, under 5 feet by several inches, they might need a little less.... For me, at 5'2, 112lbs, my requirement for a sedentary lifestyle is 1300. That's someone who doesn't work, go to school, etc, just lies around watching TV all day. The suggested amount for lightly active lifestyles adds 200+ calories.
Now, if you have fat on your body to burn, you don't need 1200, you can get by with less and burn up your reserves, but it will strain your body. Short term? No biggie. Long term? For life? That's when you can run into serious health problems.
As for starvation mode... you will know when your stomach/thighs/shoulders/whatever start collecting fat right after eating, when you are very skinny but your stomach is bloated all the time. You're small but flabby. If you're getting less than 1200 consistently, and well below it, your body will eventually think there is a famine and try to save every cell of fat it can. However, if you're only getting 600 calories a day, you're not going to gain... just strain, and after some time, you'll lose muscle tone because that is what your body starts to burn when the fat reserves are too low (your weight goes down as your muscle mass drops, but your fat stays!). But if you're not underweight, it won't happen as quick or be as extreme.
Altering your calories each day helps trick your metabolism. Having some higher calorie days allows you to enjoy low calorie and even fasting days without your body storing all the fat. This is why so many diets have cheat days. Your body prepares to save/use energy from food based on how you ate 3 days ago... to me, that means I cannot ever fast beyond 3 days, probably not beyond 2, otherwise when I eat again my body will want to hold onto all the fat because it's afraid I will be starving again the next day again!
Okay... now diet. I try to follow a vegan diet, but when I binge things don't always go as planned. The foods I eat most of the time and the foods you listed hold much more nutritional value than everything you cut out. Technically you still need more calories... but you're probably getting a pretty decent amount of nutrients without having to eat 1200 calories every day. That makes the food quality worth more, but not the caloric number, so long term, 600 calories of ANY food every day can cause problems...
And last but certainly not least, exercise! The more you exercise, the more you can eat! The more you eat, the less your body will eventually store because obviously food is plentiful, no famine! No need to hold onto extra fat! Especially if you're working out, then you're body is just as happy as could be, on 1200 calories or more most days. (Just look at the Banana Girl, Freelee, she eats 3000 calories a day!) Okay, that sounds great, but what if you don't have the appetite? Or the drive to exercise? Or blah blah blah.... that's fine! It's a process. You don't have to eat 1200 calories today, you don't have to start a consistent exercise routine today, you can gradually build yourself up to it, and you don't have to start both at the same time. My personal plan is to eat less calories than I need and ease myself into constant exercise. The more energy I feel I need as I exercise more, the more I'll start to eat. The less fat I see on my body, the more I can eat. And exercise is limitless, but if I lose motivation and stop, then I'll reduce my calories again. Once you reach your ideal weight, you want to try to keep your diet and exercise up. Then when you start to gain, just restrict a little, or exercise a little more, and you're back on track. Also, you might be gaining muscle, so don't let an increase in weight discourage you! I measure my body and calculate my BMI and barely pay attention to the scale. My waist size is my main drive. When people reach their weight loss plateau, it is because of starvation mode (or plenty of other reasons). The plateau is when you can start eating more rather than less... but without exercise, you probably won't ever get the results you want, you won't be building muscle, so you will be storing fat, and not burning much off.
Hope that helps! I used to be annoyed with the calorie requirement too, so I have done a lot of research on it. I wouldn't worry about the requirement until you reach your ideal weight or start to plateau, but I would definitely throw in some 800-1000+ calorie days. Maybe use protein shakes or smoothies (fruit was a good idea too!), that helps me when I'm not hungry or don't want to eat. Or even cheat and eat what you want instead of what's "healthy" on a high calorie day. The more variety in your calorie intake day by day, the less likely your metabolism will slow down or go into starvation mode.