your use of house A and house B is unnecessay methinks... but this sounds simular to the alternate universe theory of time travel (IE - Timeline) where we're not really going back in time, just going into a parrallel universe where we designate.

if there is only one continuum, and bob was travelling in it AND he killed mark before bob was born then it stands to reason that either mary was already preggers or mark wasnt the proud pappa...