To deal with these points in order:
1. The minimum calories that CAN be consumed is zero - i.e. fasting.
2. Too low for what? If you're talking about sustainability then yes - you can't do it for ever. But if it's weight loss/reduction in visceral fat you're after - then no, it's wearable for a few weeks. Don't forget that the Newcastle Diet (ND) is 600 calories/day for 8 weeks.
3. The lucky dieter will lose weight.
4. The dieter MAY show signs of T2 reversal or symptom mitigation.
There's plenty to research on the ND
Here for example. But not necessarily a case for worry or concern.
Hope this helps