Since you wrote the article before they offered to do a case study, it seems that the veracity of the article wouldn't be in question. And I'm not sure about c

Since you wrote the article before they offered to do a case study, it seems that the veracity of the article wouldn't be in question. And I'm not sure about conflict of interest-- it seems your interests are aligned. Now if you were claiming to be a reviewing many/all web platforms and accepted help from FB, that would be a different story. Disclaimer: I didn't read your article.