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I will try to best answer your question.
You do not have to be married or have a civil union to be able to make medical decisions for each other. This can be accomplished through a a reciprocal durable healthcare power of attorney. You have the right to collect one half of the amount of ex spouses Social Security or your own if you were married for more than 10 years. You do not automatically lose this right if you enter into a civil union. Hence I don not see this as being a major issue.
The most difficult part of your question is obtaing benefits through employer. The only way to accomplish this is to marriage or civil union. Still an exception I'm aware of is with unions. Union sometimes allow an affidavit of common law marriage.
I hope this information devices helpful and I wish you the best of luck.
I was unsure with you possibly resided in another state that may have common law marriage you could assert was created. If you wish to pursue a civil union I would advise to contact the County Clerk's office in the county in which you resigned to request the application information.