Is it legal to publish a cease and desist letter that I have received?
Generally speaking, yes.
My interpretation of your post is that you published your opinion about a business or businessperson, and the businessperson now is trying to intimidate you or deter you from sharing with others your opinion. The phrase "the actionable statements listed are clearly statements of my opinion" is otherwise unclear. Under defamation law, only false statements of fact are actionable whereas statements of opinion are not. The businessperson is not entitled to your silence.
If your criticism is about the business, the cease and desist letter sounds in unfair and misleading practices to the extent that the business is trying to conceal from the public some inconvenient information that you as actual or potential customer possess. Even if you published as a competitor, your statements would have to be untrue and misleading for these to constitute disparagement. See the Black's Law Dictionary definition of disparagement [of Goods].
For the reasons stated in the other answer, copyright issues are not a matter of concern. It is preferable to publish the letter as is. Transparency preempts confusion as to "I said, he said". By contrast, paraphrasing the letter for the purpose of avoiding an imaginary violation of copyright creates a risk of you inadvertently giving him grounds for a claim of defamation.