Does the trust have an EIN? If so, did you provide the EIN to the brokerage firm?
More importantly, you need to have the trust reviewed by a local tax professional and/or legal counsel who can actually read the terms contained therein. Irrevocable trusts can be grantor trusts depending upon whether certain powers or rights are retained by the settlor (your mother-in-law).
The optional methods you read about (in the 1041 instructions) pertain to grantor trusts and the three methods are based upon whether or not a grantor trust has a tax ID number (EIN).
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Otherwise, grantor trusts filing a 1041 get a grantor information statement while nongrantor trusts issue a K-1.
