So Plan C was not available in 2018 -- therefore they went with the closest one to what you actually paid ($80 vs $70).
What I would request is that they review all CLAIMS for 2017 and 2018 such that you are refunded any money that you were out that you were not supposed to be.
There would be a small $10 per period refund on employee contributions/premiums if they do that above and correct back all the usage of the health insurance. Since it is self-insured, they probably have a much better chance of doing so. But if they were to move you to Plan D in 2018, I'd expect them to want the difference in premiums back to 1/1/18.
I do see where you hold some responsibility --- You HAVE been paying for Plan B all year And while you may not have had notices of specific coverage (did your medical card not have the plan level/name?), did you NOT notice that your health insurance deduction didn't go from $80 to $150 at the beginning of 2018? That alone should have caused the question about a year ago...instead of "recently".
problem is your employer might be REQUIRED to cancel and enroll in what your elections actually were for 2017 and 2018.....I'd figure out exactly what it is you want. If you push too hard, they could make you pay back Plan D premiums in 2018 as long as your claims were adjudicated/reviewed under Plan D too.

