In reference to the following sentence, I am not sure if I have punctuated it correctly. Two issues: Should "the credibility of the writing and the writer immediately comes into question" be set out by commas. Second, the comma before and in "question, and this defeats the purpose of the writing." should that be a semi-colon?
If the reader identifies a logical fallacy the credibility of the writing and the writer immediately comes into question, and this defeats the purpose of the writing.