Welcome to Just Answer. I am here to help you resolve your tax and finance concerns. Please feel free to ask anytime you need extra help.
I would suggest that your best bet is to file all the missing returns first. The reason for this is that the IRS has computed your tax obligation based on only the income information they have on file. They do not yet know about your deductions and exemptions. Filing the returns will bring the matter to a head.
Once that is done, you will get billing letters from the IRS. Then you can begin to deal with this in a logical way. I recommend filing an Offer in Compromise (OIC). An OIC lets you offer to resolve the tax debt for something less than full value. It can be as low as 10% of the debt depending on your ability to pay. They can accept or reject the OIC but once accepted they will establish a payment plan with you. The payment plan can be as long as 5 years but should be something far less painful. To begin educating yourself on this submission you should go to the IRS web site www.irs.gov and review the Offer In Compromise forms and instructions. Then once you have gained a familiarity with this you can seek professional help in preparing the submission. If it is at first rejected, you can resubmit with a larger payment offer.
During the OIC submission process you will be able to state (and document) your situation. It will make them far more agreeable to working with you. They cannot take all or any portion of your SS Dbl benefit but will use it towards determining how much you have to meet living needs.