On this is-Darth-his-name-or-his-title business, isn’t the obvious explanation that “Darth” is analagous to the British Marquess, Earl or Viscount? For example, if one were to meet the Marquess of Bath or Earl Spencer (Princess Di’s ghastly brother), one would address them as “Lord Bath” and “Lord Spencer”. Likewise, I notice that in the later (ie, earlier) Star Wars movies Darth Vader, Darth Tyrannus (Count Dooku) et al are addressed as “Lord Vader” and “Lord Tyrannus”, clearly indicating that Darth is a rank of peerage that is not a form of address.
This would also explain the fanciful names the various Darths have – Tyrannus, Sidious, Maul – in the same way that Donald Smith, founder of the Canadian Pacific Railway and president of the bank of Montreal, was raised to the peerage as the Earl of Strathcona and Mount Royal. “Dooku” is presumably the equivalent of “Smith.”
Thus, when Obi calls Vader “Darth” in the first (ie, fourth) Star Wars, he’s like those American tourists who hail the Duke of Devonshire as “Dook.” Or alternatively, being unfamiliar with Sith style, he assumes “Darth” is the first name in the manner of Duke Ellington or Count Basie. Or Gaylord Ravenal, who is the raffish southern n’er-do-well in Show Boat rather than an actual gay lord.