Your message suggests that your former spouse worked for a government entity. If so, the pension arrangements are probably not subject to ERISA and the common knowledge that goes with the body of law under ERISA. The plan terms, which may be statutory, will determine survivorship rights. You will not be able to get your answers based on general knowledge and expertise, only specific attention to your situation and the related plan.
It is possible that the employer was not a governmental entity, but a nonprofit organization instead, and ERISA may apply.