I think that Jane keeping her kings in the back row allows her to be in control, and allows her to have sort of a fortified front. On page 88, Holden describes the memory where she was likely abused by her step-father, so as a victim of abuse, the checkers strategy is her way of making sure she is in control, and that no one can literally sneak up behind her perhaps. Just a thought! The ducks in the lagoon can fly south, but Holden has trouble understanding where they go since he doesn't "SEE" them fly away or end up anywhere else. I think this is very similar to his brother Allie who died (or disappeared) for reasons that Holden cannot fathom (leukemia -- cancer), and since Holden didn't even go to the funeral, he doesn't even know where Allie is. He never saw him go away, so he feels like this is unresolved. In a similar manner to Jane, I suppose both ideas are related to control. Holden wants to control people's ability to come and go, and wants to invite people over, but struggles with not being able to make them stay (see: Sunny, Stradlater, Ernest Morrow's mother, etc...)

In my opinion, Holden is wondering where the ducks go when they have no home/lagoon. I think Holden is seeing this in himself, like he is wondering what will happen to him when "his lagoon freezes over"/where he has to go when he has no home