Before you hire an attorney you might just want to talk to the ex-spouse and see if the plan has a benefit for your mom as an Alternate Payee. It is possible she is due the money but the plan provisions don't allow them to be paid until some triggering event.
The ex-spouse might be able to say she got paid and show that it happened and your mother forgot about the payment.
A simple conversation won't hurt and could save you costs. If the conversation doesn't result in what you think needs to happen then you can hire the attorney.
The way I understand it, you could do an "other" attachment to the 5500 with a statement that an election is made under 4972 that no taxes are owed. This would start the SOL just like filing a "protective" 5330 to start the SOL.
The court rejected the argument that NOT filing a 5330 is the same as filing a 5330 showing $0 owed.
There was a good ASPPA article on it that I read earlier today, I'll see if I can find it and post the link.
The current version of the IRM lists the 5330 in the chart under 25.6.1.6.4. I have no idea if or when this recently changed. What is less clear to me is why the 5330 is not covered like the 5329 has been forever in 25.6.1.9.4.3.6. It is certainly a form reporting more than one item of tax.
25.6.1.6.4 (09-20-2016)
Statute of Limitations Chart for Tax Returns
The filing of the Form 5330 starts the running of the statute of limitations, except for the section 4975 excise tax, the filing of the Form 5500 starts the running of statute of limitations for section 4975 excise tax. It is 3 years if the information is disclosed and 6 years if it is not disclosed on the applicable form.
25.6.1.9.4.3 (04-01-2007)
Forms Reporting More Than One Item of Tax