On the first questions, medical benefits are life time benefits assuming you did not close them out in a settlement. Accordingly, if you had admitted work injuries and the insurer paid for some care, the insurer is on the hook for future medical care if it is reasonable and substantially related to your injury. Settlement? It depends. Termination is a good time to consider settlement. Be sure you talk to a lawyer about settlement and do not do it on your own.
You should still have medical for your back even if you were terminated.
We give free general concepts to be helpful, but you should give ALL your facts to a licensed Attorney in your state before you RELY upon any legal advice.
As long as your back injury is an admitted claim and you have not settled the issue medical treatment for a work injury is open for life in Minnesota. So, you should be able to get medical coverage for you back if it is an admitted work injury. Your termination would have no impact on your workers compensation claim when it comes to medical coverage. With that said, the insurance company may still deny certain treatments if they feel the treatment are not reasonable and/or necessary. If they do deny any treatment you have the right to get an attorney and fight that decision.
Insurance for losses from an Industrial Injury remains in force based on the date of the injury, not on the state of employment.
You write "I have been on work comp a few times" which I think means you have submitted a few workers compensation claims. IF THOSE CLAIMS were resolved keeping Future Medical Rights open (and doctors wrote you would have flare-ups requiring additional treatment in the future), then yes 'comp still covers' the back problems.
HAVE YOU SEEN THE MINNESOTA DEPT OF LABOR/INDUSTRY SITE? it's pretty informative, easy-to-read: