Should publishers of scientific papers such as Elsevier be regulated by the government?
According to the most basic theory of economics, the consumer surplus
  is maximized if and only if the price is equal to the marginal
  opportunity cost.


You are mixing agents in using these concepts. Consumer's surplus (or utility) is independent of the producer's marginal cost. Paraphrasing Investopedia's explanation, the consumer surplus or utility is maximized when the purchase of an additional unit equals the marginal benefit the consumer obtains from that additional unit.


  Should publishers of scientific papers such as Elsevier be regulated
  by the government?


No. Doing so could prompt the publisher to reduce the quality of its product or --in an extreme scenario-- close operations altogether. The existence substitute publications which are available for free further weakens the need for governmental intrusion in a private entity's decision-making.