Neat mechanism to give player a grace period to work with, secure the zone, then expect a counter attack/migration!
Much more interesting than vanilla "kill once and ignore"

With the way you generate a new contender for the zone, by cloning one from a neighbouring territory, the next migration could come from basically any neighbouring chunk (due to patrol)? This is a really interesting in terms of "I have some time to secure this zone but dont know the direction of attack"

"Kill the migrant to prevent recapture attempt" - is that territory considered back in the grace period, and eventually due for another migration attempt?

Overall really excited to try this out, thanks for publishing and the solid description.