What are options for LO that is on hospice palliative care, cannot be left alone, but refuses all suggestions of care?

Whoever this Social Worker is, and however she is involved, it is clear she is about to open APS case. In my book this may almost be best because at present you have no proof she is demented and incapable of making decisions for herself. I honestly think that you should tell Social Services that you cannot override her unless someone says that he has dementia, and can no longer make her own decisions. Tell them that if they would like to assist you with emergency guardianship you will be happy to serve in that manner and place her in care if APS says she is currently a danger to herself.
I honestly would feel as helpless as you do. It is very difficult when you are 5 hours away, as well. And then add on Covid-times. Your hubby can either place her against her will taking the chance they will deem her competent and move her home at her request, or take this route. I can't figure another.

I don't like to suggest something like this, but if APS becomes involved, she may lose her choice of options.    Who is the social worker, and what's her role?  I.e., is she part of the home care team?  

I have strong feelings about social workers, and refused to allow 2 of them to come after they became aggressive.   One insisted on seeing financial data so she could "recommend" an appropriate placement or solution.  I told her my father's financial situation was none of her business and she would NEVER get access to it.  She just couldn't accept that I handled that and didn't need her meddling.

Another took a similar position, but couldn't tell me anything that I didn't already know.    We'd already been advised of options; I didn't need to hear them a second time.

Is your father a Veteran, and if so, is he enrolled with the VA?   From my experience, their social workers are much more adaptable and compassionate.

What I would do is play along with the social worker; get her to find some local places but don't tell her you're pursuing your own goals.   

Others have dealt with the issue of a parent not wanting to move; sometimes it's an intractable issue, and I do understand a parent's position on that. 

If she's threatened by a social worker, that would only exacerbate the situation.   Is there a way though that you can leverage the SW's position, and explain to your MIL that you and your husband could lose care of her, and/or that she could lose all of her independence?  Or that she might end up someplace that wasn't suitable?   I don't like to scare people, but sometimes a little bit of it could prompt some to consider alternatives.

Do you think that taking her to see care facilities might help?  Unfortunately, that's not as easy as it could be given the pandemic.

I don't really have good solutions but just wanted to offer some support.

Consult with her PCP about determining her competence to make reasonable and safe decisions and to evaluate her ability to care for herself. Both sound poor. If PCP determines incompetence, your husband's durable POA may be able to take over and hire help at whatever price necessary whether GIL likes it or not. APS might also be able to guide you on what is required to declare incompetence.