What do you do when Mom has the right documents in place and nieces and nephews start showing up attempting to get her to sign a new POA?

Suggestion: Write a letter or have an attorney IN YOUR MOTHER'S STATE write a nice letter pointing out the definition of undue influence. Discuss federal and state statues that address undue influence. In particular, mention that under federal law undue influence that deprives an at-risk person of the use, benefit, or possession of anything of value is considered criminal exploitation and can be considered a class 3 felony. This might give them the heebeejeebees.

Oh, and send a copy of the letter to all of them stamped Return Receipt Requested.

Do you have enough gumption to ask these vultures why they feel they ate entitled to anything of your Moms. Children always come first with inheritance. Without a Will nieces and nephews are pretty far down when it comes to who inherits.

If Mom has been diagnosed with dementia then she can not sign any legal papers or contracts.

You parents cannot change their documents if Dementia is involved. Maybe placing them in to a nice AL would help keep the vultures at bay. Moms asked me who was allowed to take her out of the facility. As POA you can list who is not to see her. These people are not good for her or Dad.

She has a HC POA and a Trustee, but does she have also a POA that has the power to act for her in terms of placement and etc if she is unable to make decisions for herself. Does the Trustee papers include this power to act for your Mom in all instances?
If this is the case, I can assure you that she is in better shape with a Trustee, as this takes her Trust out of the hands of those who are after any money. They cannot replace a Trustee very easily unless they prove fraud. Trusts are not public documents.
You should speak with the Trustee. Whomever has this important role can then go, using funds from the Trust, to see a Trust and Estate Attorney.
Is your Mom currently competent? If not she cannot sign ANYTHING that could hold up in court. If she is competent it is unlikely she would have resigned as Trustee of the Trust she created as TrustOR.
Anyway, in all of this, your Mom's funds being protected by a Trust, it will be very difficult for anyone to meddle with. Nevertheless, with nefarious people descending after money the Trustee needs to lock in whatever is needed to stop their activities.
Again, know that getting a POA requires a competent person. And even then, a POA is WORTHLESS to do ANYTHING AT ALL with a Trust. Only the Trustor and the Trustee have any power over a Trust. Hopefully Mom's money is safely protected within.
And again, have the Trustee contact a Trust and Estate attorney for advice. Realize this is just a Forum. While I was both POA and Trustee of my brother's Trust per his request, having served in this capacity makes me no expert. When you are dealing with an elder's safety and their assets you need to have expert advice.

It’s disgusting. Theres 10 of them and they are showing up one right after the next - closer and closer together after not contacting her for 5-8 yrs. We know they are planning something… they won’t even wait until she dies to try and get their pockets lined. They already tried to put her in a nursing home once before. She’s not even THEIR mother! Special place in hell for these vultures.

Your Mom needs to be cut off from these visitors. I had cousins who attempted this. We did not allow visitors without one of us being present (this was done after a niece visited and took a full-length mink coat from my Mom. We did not take her to her sister's house without one of us present because her sister asked her repeatedly to go to the bank and withdraw money for this same niece. She also advised my Mom to make her POA because she was a little older than me (she had already filed bankruptcy twice). After my Mom died she tried to go to probate and be appointed executor but everything had been filed. She called and asked me what jewelry mom had left her. Total vultures.