Although the Americans refer to him as Lord John Marbury, Marbury describes himself as "John, Lord Marbury" and gives a list of his other family titles. This means that he should always be addressed as Lord Marbury, not Lord John. The two are mutually exclusive. The only way he could be Lord John Marbury would be if he were the younger son of a Duke or Marquess, and thus not due to inherit the actual family title.
However, in referring to himself he would also say "Marquess of Needham and Dolby" *before* saying "Earl of Croy", as Marquess is a higher rank, and titles are given in descending order.