OJ, I don't think you need Power of Attorney to keep MIL out of house as you are the householder so you can say who comes and goes.
As for getting a POA for other things, I am afraid that the rules on POAs changed a couple of years ago and I don't know the up to date rules. I'm a litigation type of girl - always up for a good argument .Have you got a friendly local sol who could give you some advice? If you phone up and ask to speak to someone int he private client department, they can outline the position to you over the phone (for free ) and you can decide if it's a good idea. I'm working at home today so can't access my books but am happy to look it up for you when I am in next week if that would help?
Have a good afternoon at the school.