Base umpire is totally wrong, on a number of levels. First, he has no business calling the batter out whether he interfered or not....this was the PU call. BU should've conferred with his partner, informing him that pitcher disengaged, therefore play should've continued....BU and PU can discuss whether batter intentionally allowed himself to get hit ( I suspect that to be unlikely ). The play should've been allowed to evolve, with runner safe on the steal, batter remains at bat. However, if the PU killed the play before runner touched home, I'm not sure if this qualifies as a "correctable error" or not.
As far as adding a ball to the count, that is totally wrong, since pitcher stepped off, making him just another fielder.
Also, technically, if there are two out and the batter interfered on a play at the plate, the runner would be out, whereby the batter would lead off the next inning.... less than two out, batter is out and runner returns to third.