Luminosity!Bella's shield also has some sort of an enhanced-immortality power

If you want to phrase it that way, I suppose it's an okay label. When the existence of her mind is in danger, the shield will force her body into the minimum adequate configuration for her mind to go on existing, but it's not omnipotent. She would have died if it had been a long time since her last feeding, or if her pieces had been scattered over a square mile or more before being set on fire, or if she'd just been ignited again after the flames went out.