A power of attorney can be revoked generally, but it depends on the language of the document. Plus, with the patient having dementia it could be possible that obtaining another power of attorney document by prove extremely difficult because the patient may not have legal capacity to execute a valid, lawful signature. It can be legal for a person who has power of attorney not be a care giver. Many times when a medical power of attorney is executed it only grants them rights to medical care decisions and they are not required to give the care. However, as always, whatever language in the document controls the situation. But to answer your questions, yes power of attorney can be revoked provided it does not say differently in the document, and yes it is legal for the power of attorney to not be the caregiver provided there are not stipulations contrary in the document.
