As I look upon the MWI: every event is a branch point of a particular universe (i.e. the universe before the event) to a set of universes after the event. Every universe in the set has another outcome of the event as it's start point. All universes are to be considered as objectively real, all outcomes of the event do occur, all in another universe and the different universes cannot interact.
You descrive an event with one particular outcome: the annihilation of a particle-antiparticle pair. The MWI is not related to this situation, your universe is the same before and after this event because you do not consider different outcomes of the event, you assume an event with only one result (annililation).
Some further remarks:
1) Your phrase "position of the universe" does not make sense, because position is measured ín some universe. For a universe to have a position we have to consider another universe that contains it.
2) The MWI is extremley absurd. When there are 10^80 paricles in the universe and each particle has 10^40 events per second, then after 10^17 seconds there are (10^80^40^17 = 10^54400) universes. MWI defies common sense also in other respects.