Your mum is an adult and as such must bear a degree of responsibility for herself. Unless you are extremly wealthy you cannot sustain keeping her indefinitely and she needs to make some effort to pay her way. She needs to know what she is entitled to and she also needs to know how to earn her own money. Studying is what you do between work and chores if she wants to study full time she can become a nun. It sounds harsh but if anything were to happen to you she would not have the life skills to cope.
On the other issue she has the right to believe whatever she likes but she does not have the right to terrify your children. As a JW she thinks she is teaching them the truth and will not be able to help herself. You can explain to your children that people believe in many strange things and granny too has some funny ideas. Sadly some can be a little scary so if granny tries to talk about religion then say I'm sorry but that is not something I want to talk about. Reinforce to your mother that religion is off limits and remind her that to continue to live together she needs to respect your wishes.
I would investigate housing in any case and look if there is any alternative accommodation and move her towards the goal of independent living if this is achievable otherwise there is a distinct possibility that your current arrangement will crash and burn.