If you are seriously under your calorie goal once or twice a week, I probably wouldn't worry about it. If you're under your calorie goal every day, week after week, that's not going to be healthy long term. You just can't get all the nutrition you need without a certain amount of food. So, it's not just calories that are going to be low.
I think it's what you do consistently that's important. I have days that I don't even eat 900 calories, but there are days that I will eat a bit over maintenance too. Both of those are just occasionally. More consistently, I am able to stick to my 20% deficit goal of around 1270ish.
I will often feel a little more hungry than usual the day after a very low hunger day, but who knows what the real cause is. I definitely wouldn't say that has happened every time.
I think the general thinking is that by the end of the week, are you at a safe calorie restriction? I think up to 20% deficit is widely considered the maximum safe amount, but I did see up to 30% mentioned at http://www.ruled.me/keto-calculator/
There, he sites a study that suggests anything more aggressive than that can have negative effects on your dieting. I haven't looked at the study. I have no idea what it suggests.

becoming so hungry that I fall off the wagon and have to get back on. Yet again.

Generally speaking, that's not likely to happen. Yes, your hunger may increase, but hunger in a primarily fat-burning state is different from the "hunger" experienced in the primarily sugar-burning state. The latter is generally the result of a drop in blood sugar. It's that "must eat NOW!" feeling and the desire to shove in your face whatever you can get your hands on. The former is completely different. Yeah, you're hungry. Your stomach may even be growling, but you can often still turn down food if you need to. A little discipline to order the fatty steak -- without the potatoes -- or wait until you've finished cooking your meal, and go after the fat before the lean parts of the meat, and you'll be fine.

Yes, you might go over in calories at first, but a) your body will regulate after a while as long as you let it (and really listen to whether you're hungry or not), and b) it's not about the individual days, but the averages over time. Any day that you have that's over your calorie goal, before freaking, look back a week or so, and you'll likely find at least one day that's under by about the same amount (or several days under by less, but add up to about the same). It happens, it's normal. Don't worry about it as long as you're not using it as an excuse to eat a bunch of extra stuff when you're not hungry.

Don't worry about the appetite bouncing back - as long as you stay in ketosis.

Have easy to get to keto friendly foods around. I keep pepperoni and block parmesan cheese close. If you do get hungry those foods will hold you to mealtime.

If you're hungry eat - the right things.

Calorie restriction is important. Staying in ketosis is more important. And for most folks once you experience the benefits you really don't want to go back. That is the way I've been able to survive Church potlucks, anyway! :-)

It's a wonderful new normal! So glad you've made it! They usually will even out with hungrier days throughout the month, but 500 calorie days is too low. Remember part of the mindset is to use what we eat as the fuel our bodies need to function optimally. If your not giving it enough fuel consistently, you will eventually run the motor empty, so to speak. Just be cautious. But enjoy not being hungry! It's an amazing place to be!