My mom was just diagnosed with vascular and/or Lewy Body dementia (more tests, probably a combo) at the late mild, into moderate stage.

Her anger is part of Lewy Body dementia. There will be no reasoning. It is part of her disease that makes unsafe choices. You just need to tell the social worker that she has no social supports upon discharge and her doctor already indicated what she needs. This is an unsafe discharge. It is best to stay away and let state probate take care of her.

I can tell you, after serving as POA for a capable, agreeable, quite lucid, accomodating, organized and exceptionally loving brother, that it is an enormous job. You are not living there, and while I lived only the length of my long state away, it made everything much much more difficult.

If you are thinking to try to act for a disagreeable, uncooperative, albeit demented elder I would caution you against attempting this.
I would not visit if I was not welcome.
I would report her to the APS and refer them to her MD and allow state to take guardianship if she is in unsafe conditions.

My advice for what it is worth. It may sound cold, but what you are attempting will prove utterly impossible, especially if she is uncooperative. Once taken up for a senior, it is very difficult to resign a POA without accusations of abandonment. Don't do this. Sorry, but just don't.

Please know that vascular dementia is the most aggressive of all the dementias with a life expectancy of just 5 years and Lewy Body is the second most aggressive with a life expectancy of just 5-7 years, so it is very important that your mom gets her "ducks in a row" sooner than later.
You have to now remember that your moms brain is permanently broken and will never get better only worse, so you can't take what she is saying so personal.
I can only imagine that the extended family and friends(who you say are "very worried" about your mom)that are there, would love to have you come help them get your moms things in order whether she wants you to or not, as I'm sure they will be more than grateful for your input.
This journey with your mom won't be easy, but you're now going to have to step up and be the adult/parent in this situation. Someone has to, as your mom can no longer do that.
I pray that God will give you wisdom and discernment in this very tough situation.

Don’t go and let her fail. When the emergency happens, call the ambulance (don’t tell her this is happening) and when she gets to the hospital, tell them she is an unsafe discharge.

It’s too late for your to get POA. That ship has sailed. Be grateful for it.