FERCCA does not give you a choice about Social Security coverage. If you should have had Social Security coverage during your Federal employment, then you must have Social Security coverage in addition to your Federal retirement coverage. You have no choice. If your agency incorrectly put you in CSRS when it should have put you in CSRS Offset, it must correct your retirement coverage to CSRS Offset. You will not be able to get the full amount of the refund you were expecting.
Your previous agency should have sent the Social Security Administration a record of your earnings during all the years you should have had Social Security coverage. All of the CSRS contributions you made during those years that are not needed to cover your retirement costs were transferred to Social Security. Your refund was based on the retirement contributions that should have been withheld from your pay. It did not include amounts that were properly withheld, but erroneously considered retirement deductions rather than Social Security taxes.