Hi Holger,
If VM remains at 24V then this would be a case 3 scenario. I can see how the figure creates confusion stating "allowable" and showing VM only up to 14 V when in reality VM can go up to 45V as per the Table 9, case 3. I have noted this and we will update the datasheet to better explain this part, thank you for your input.
For the question when VM falls below 5.9V, and low side FETs driving, yes you can add a backup capacitor or just place a bigger bulk capacitor for VM + the diode. It would not damage the DRV8701 device. I hope this helps, thank you.