Model positioning during multiple close combats after removing models
This exact situation is covered under Incomplete Ranks in the Close Combat section of the rules (pg 49.)


  If the unit is fighting to its rear (or fighting to both flanks) some
  enemy models might not end up in base contact because of the models in
  the incomplete rear rank. In this specific situation, the models can
  fight across the gap, even if not physically in base-to-base contact.
  In reality, the chargers would not have stopped one step away from the
  enemy and would have moved in to continue the fight — treat these
  models as being in base contact with the enemy.


They count as being in base contact even though there is a gap.