Read
http://www.guinealynx.info/weigh.html and
http://www.guinealynx.info/teeth.html
Only a necropsy could tell you for sure but honestly it sounded like she had maliclussion that was ignored and she starved to death.
She wasn't 'taken care of' at all by our standards.
Bottom teeth can fall out due to injury or scurvy. If she wasn't eating, scurvy is likely.
You can't examine the teeth properly without proper tools or dental x ray. Usually the problem are with molars. You are not a vet so you wouldn't know what to look for. Hay alone is not a cure all for teeth issues if they are genetically prone to them.
Teeth issues can be secondary though, so she may have had more issues going on. Pet store pigs are often inbred and have poor genes. But that doesn't mean she couldn't have gotten better with proper medical treatment from a conscientious owner.