I think you issue is highly unique, and more a question for an accountant and/or a tax attorney. Although the social security benefits that you would receive are retroactive, they are taxible in the year you receive them. This I am positive of as I was on Leave Without Pay (LWOP) from September 1, 2007 until my case was finalized on September 30, 2008 (My FERS Disability Retirement was approved on June 20, 2008; therefore, I was in an interim pay status until September 30, 2008). I received "back pay" at 60% of my high-3 salary from OPM for the period I was on LWOP. Health insurance premiums pre-paid by my (former) agency, federal income taxes, and gross interium pay was deducted from the back pay (state taxes WERE NOT deducted). Although part of that back pay covered the LWOP period in 2007, it was fully taxable in 2008.
The FERS Disability/Social Security offset issue opens up a whole can of worms and most likely would require an amended return (but best answered by an accountant).
Also, each state has different tax laws relating to Social Security payments which may require an amended state return.
Another point, in almost all cases, Social Security payments are via direct deposit NOT paper check; therefore, if your direct deposits go into a checking account you will need to carefully keep track of your account so that the retroactive Social Security payment can be moved to another account so you do not by error spend it.
Also, even though there often is a direct link between FERS Federal Disability Retirement and SSDI, DETAILED Social Securty issues are generally not widely discussed on this Message Board; but, I believe there is a Social Security Message Board for SPECIFIC to Social Security issues that can't be answered here. This, in addition to the uniqueness of your situation is most likely the reason that you have not gotten much feedback to your question.
Finally, in most cases Medicare does not start until one has received SSDI for 24 months; therefore, I'm not sure if, or if not this raises implications due to retroactive SSDI payments, and issues related to FEHB as Medicare becomes primary (just a thought).
Hope this answers some of your questions.