How is the exit behavior controlled after a plot is complete in CollectEarth?

You cannot control it, it always does that.

If you need to change it so that it doesn't do anything after the data is saved then you will have to modify the earth.js file inside the CEP in the folder earthFiles/js (as always open the CEP file with WinRAR/WinZIP/any compression software) :

if (json.validData) {
    showSuccessMessage(json.message);
    forceWindowCloseAfterDialogCloses($("#dialogSuccess"));
}

So you will need to comment that forceWindowCloseAfterDialogCloses method by adding a couple of backslashes like so:

//forceWindowCloseAfterDialogCloses($("#dialogSuccess"));