I think Schwab should have created a new trust with it's own EIN..."
The trust should have applied for and received an EIN but that process was likely not Schwab's responsibility.
You can contact Schwab and request that they issue a corrected 1099-B with the trust's EIN but, if that is not a possibility, you can file a 1041 with a statement that the income and gains shown on the 1099-B belong to the trust and your mother received them as nominee.
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