The movie industry hasn't changed all that much over the years in one central aspect: making money. Shorten the movie, fit in more showings, more ticket sales, more money. It's not at all difficult to see the Universal suits saying "Why does Dracula visit Mina twice? Cut one out and we can fit in another showing per day." So someone is assigned the task of removing one visit to Mina. Maybe Browning was involved, maybe not, but it makes hash of the continuity, and even Browning had to have seen that. Maybe the same fate befell Lucy's staking--"Why two stakings? You have one at the end, that's good enough. Cut the first one out." That seems like the most natural explanation for the complete omission of one of the key scenes of the entire drama. Follow the money.

Yes, conjecture, but it's better than an elaborate argument based on a misrepresentation that the script has only one, not two, visits by Dracula.