It is my understanding that the work comp offset only applies to Social Security Disability payments and does not apply to SS Retirement payments.
Workers' comp money paid for personal physical injuries is not subject to federal income taxes.
You do not need the store's cooperation in order to file criminal charges against him. You can file a police report and get a warrant issued by yourself. Civilly, you need to promptly pursue workers' comp if you were injured. I suspect that suing the employee would be money wasted due to his lack of assets. Good luck.
As a volunteer you may actually be covered under an insurance policy. Don't assume they know. But I can assure you that if they and you lie to the insurance company about your status then you could be charged with insurance fraud, which is probably a felony that may require you to do some time in prison upon conviction. I would be careful about following through with their ill-advised plan to claim that you were a visitor if you were not.
You should get legal advice from a local Penn workers comp lawyer before you give a recorded statement. I have written an "AVVO Legal Guide" on this topic that explains the dangers of the recorded statement. You can find it on my profile.
Give your injury some more time to heal before you get too negative about your professional future. Your back has not even fused yet. You may have a better outcome than you expect. I know a surgeon who has a lumbar fusion and he still operates several times a week (standing up for hours, leaning over the patient) , and in the winter he jumps on a plane on the East Coast and flies to Colorado and goes skiing in the Rockies for fun.
Mr. Borah gave you great advice.
When I started practicing law over 20 years ago, I had the good fortune to work for a gentleman who had graduated from law school nearly 40 years earlier, and he had handled thousands of cases successfully. He was a great mentor. He had a few rules, too. One was "Never, ever, be the third lawyer on a case, unless one of the first two lawyers died during the representation. " This has proven to be a good rule.
Why is that? Well, you may not like...
The first 7 days of disability are the "waiting period" and you can take sick or vacation days for those. But they cannot substitute your sick days, etc, for workers' comp TTD benefits. If you need further info, get a free consultation from a workers' comp specialist attorney.