It almost sounds as if your MIL could be suffering from depression herself. What may appear to be laziness could be her just not caring enough about herself to do anything. Depression in older people can also mimic symptoms of dementia or Alzheimer's, but with treatment the confusion can get better.
If she truly is having some beginning dementia, she may realize that "something" isn't right, and that could also be contributing to depression.
And yes, if someone is depressed enough, they won't even care to get out of bed to use the bathroom.
"The 36 Hour Day" is a GREAT book and will help explain dementia and the behaviors with it. I would also strongly recommend that you have your MIL checked over by her doctor; check for medical causes for these behaviors and then also screen her for depression.
Here is a good site that helps explain depression and dementia in older persons:
Depression in Older Adults and the Elderly: Signs, Symptoms, and Help