After one Coalition conducts battle and resolves all territory control, another coalition causes adaption and placement of new invaders in territory's that already concluded battle. Do you initiate another battle phase again after all 5 coalitions have taken a turn in this case?
Yes, there can only 1 Faction in each Territory. Earth Defenders or Invaders
Is Overrun only determined during step 9 "Territory Status"? I ask because during step 15 "Drop Ship" an invader can be placed which would exceed a area's defense value.
Yes, it is only checked in the Territory Status.
However, dropships counts as units so 1 dropship and 2 invader units will overrun a defence 3 area
A territory is Overrun if it contains a number of invader units and drop
ships equal to or exceeding its Defense value and it contains no coalition