Although I'm not an expert on US visas, your non-resident mother would probably NOT be allowed to return directly to the United States. However, she could go spend 14 days in Mexico or the Caribbean before returning to the US, but given all of the visas, her nationality might make that not be as straightforward as it would be for others...

She's probably OK for travelling to the Netherlands, especially if fully vaccinated. But it looks to me like a lot could go badly wrong, given the residence situation of your mother.