Ask for the Summary Plan Document for the retirement plan. There must be one by law.
Relative to FICA, back pay IS subject, as I noted. It can be all recorded in the year paid or, preferably, allocated by the employer to each year to which it applies. If the sum is large, and it is allocated over a substantial period of time it MAY make a difference in your future benefits, but we would have no way of knowing the actual amount. The employer must match your FICA contributions.
I'm an HR/Payroll professional, not a lawyer, but maybe an Enrolled Agent could give you more complete advice before you consider an attorney.