I think it's simply self evident that it was a name when that script was first written and then it became a title during the later expanding and revising of the series as it developed. After that first movie, there is never again a single time when a Sith is referred to merely as "Darth". Not even once. Referred to merely as "Vader", yes. But as "Darth," no.

It's not a detail that can't be rationalized and made to fit with the later movies, just like you have done canofhumdingers. Your approach is perfectly valid. But it's just like Luke's clear attraction to Leiah in A New Hope (and the intense romantic moment between them that was deleted from ESB) is evidence that the whole sister angle was an innovation of RotJ and not something that was already planned from the beginning.

Earlier in A New Hope Obi-Wan says Darth Vader was his name even before he turned to the dark side. And don't counter by pointing out that he was lying to Luke; remember ESB and RotJ didn't exist yet (and if you're going to try arguing that Lucas had all the twists planned out from the very beginning instead of making it up as he went, don't bother; besides the few examples given in this post, a list of others could also be given to further discredit the "saga was all written from the very beginning" fallacy)

Time to get back to the subject of being blase about the new trilogy though (the true topic of this thread)