Principello was not added to the Uranium fleet until after Volturno was lost in October 1913. According to the Nieuwe Rotterdamsche Courant (3 Jan 1914), the former captain of Volturno, Francis Inch, departed for Genoa for delivery of the newly acquired ship Principiello[sic], The same newspaper reported the ship’s arrival at Rotterdam under its former name, Principe di Piemonte, on 26 Jan, and reported an anticipated first Uranium Line sail date of 5 Feb on the company’s Rotterdam – Halifax – New York route. The Morton Allan Directory of European Passenger Steamship Arrivals lists the arrival Principello (its first arrival for the Uranium Line) on 8 March of that year, so the initial sailing was probably delayed by about two weeks.
It's an interesting theory that the mystery ship was Campanello. As a point of reference, Volturno, which (as you mentioned) was operated by the same company, was also nominally a 13-knot ship. If one looks at sailing and arrival dates extracted from Dutch newspapers, Volturno most-often took 12 or 13 days for eastbound crossings. If Campanello had encountered ice along the way, a 14-day eastbound crossing would not be unexpected.