The key to understand this kind of experiment under the MWI is to remember that if you erase (or never register) information specific to one of the branches of the super-position, then you can effectively merge two branches just as easily as you had split them.

The delayed quantum erasure is too complex for me to analyze now (it has a lot of branches), but the point is that when you make two branches interact, if those branches have not acquired information on their path, then they can be constructively re-merged.