First, can you tell me, you were earning $25k per year, when it should have been about 25% less than that? (Such as 19k?) I think that is what you are saying.
Once the correct payroll is reported to IRS, and then to SSA, you will want to get your paperwork and ask for your earnings record to be corrected in your SS file. While you will still be over the earnings limit while collecting that dependent survival benefit, and therefore not due ALL of the SS you received, the overage should be less, thereby increasing what you were due from survivor benefits. While there is generally a 3 year (approx) time limit to correct any particular SS year of earning, exception do exist, particularly if it is employee mis-reporting your earnings, so push for that.
As such, I believe there is a good change of getting your lost survivor benefits paid to you.
As for your employer, perhaps consider trying to negotiate with them, a compromise. For instance, if you DO have, say, $3000 in savings you could use to pay them back, consider offering 3000 now, in full and final settlement of the WHOLE claim (thereby saving you 8000)... perhaps they will bite, because a bird in the hand is better than 2 in the bush. There are many reasons they should realize could cause them to NOT get paid, if they have to wait for you on a payment plan. For instance, let's say you say you can afford $50/mo. That will take 224 months to pay off. Lots can happen, you could have medical expenses that cause you to not be able to pay the $50... they will have to hound you.... you could just decide you are going to breach the payment plan.... you could become sick and simply forget to pay... or (gosh forbid) you could pass away - and pay nothing more. It is reasons like this that parties, when owed a larger sum, will often take a smaller sum even if way less, if it is paid immediately. If something like that happens, you will want to make sure the settlement agreement is signed by all, in writing.