It appears the final touch work on Diagon is really accelerated lately, so the June 1st (or maybe even a week before) for soft openings may happen. The small IOA side of the expansion seems the most behind at the moment.
It appears there some sort of internal mini rush (aka soft bet) to get Gringott's soft opened before the seven dwarves mine train's official grand opening. Competition is pretty good, either way it very well may happen.