My mother-in-law is in late stages of dementia, I am her POA. Can I get in trouble if I take her to ER for UTI and say we can't care 4 her?

Good insights have been give to you. You may have to go through a "protocol" to resign as her PoA, if that's what you want (look it up for your home state). If she hasn't been receiving any mail at your home, then it can't be proven that it was ever her residence in the first place (and the hospital discharge staff needs to know this). Tell them she was evicted and her staying at your home was a temporary aid -- she has no home. Follow all the other advice given. I wish you much success in working through a solution.

This is what we in kindness call the "ER Dump" and sometimes, unfortunately, it is the only way. You will have to ask fro Social Services right away and say that you cannot safely care for her in the home and that she is a danger both to herself and to you and cannot return home. I would say that there are, yes, symptoms of a UTI, but I would also say that she needs a thorough assessment for her mentation. Include the combativeness; say you are afraid. Include the fact you are facing homelessness and she must have placement. Do not argue with them about anything as they will promise you anything. They will tell you "We can get your help" or "we can make this work". They can't and they won't.
If your grandmother is assessed competent then she will be discharged if she requests it, but that sounds unlikely.
Tell them that if she is returned home you will refuse her. They may tell you that you cannot serve as her POA. If that is the case tell them you would like the State to become her guardian. That is the most certain way to her safe placement.
I am sorry you are faced with this, and I am sorry your MIL is. If this is symptoms of a UTI it will have come on suddenly, but it sounds more to me that you are looking at a dementia you are no longer able to care for. I wish all of you luck and hope MIL gets good placement after diagnosis and treatment.

Yes, call 911 to take her to the ER, then meet them there and state frankly that this is a social admit and unsafe discharge: You are disabled and cannot take her, end of, and she can't even go home since there is no home. They will have to figure out a place to put her.