I tried to make some guesses but there are too many unknowns.
For months where entitlement to SSDI and W/C overlap, your husband and his dependents cannot receive more than 80% of his average current earnings. The SSDI claim cannot pay retroactive benefits more than 12 months before application, regardless of onset date.
He will need to be sure that all his worker's compensation data has been provided to SSA. If it wasn't all provided at the initial interview, it is doubtful that it is all included in the electronic file. SSA employees don't go looking for work on claims that have been denied. So, the payment center may pay initial benefits without current W/C data which means they may apply the offset until the documents are submitted.
SSA needs to determine his monthly SSDI amount before offset based on lifetime earnings; his average current earnings at the time of onset of disability; the months monthly W/C was paid; what the $30,000 settlement actually represents in terms of future benefits and for what months.
I kept trying to come up with examples, but there are too many variables.
Be sure the payment center has all the W/C data. If they ask for it, submit it even though you think you have submitted it previously.