batch duration issue

Yes it overwrites it . that is the whole purpose of it . to disable that please add this given code in your wplms-customizer.php file in wplms customizer pluign :

add_Action('init',function(){
    if(class_exists('Wplms_Batches_Filters')){
        $filters = Wplms_Batches_Filters::init();
        remove_filter('bp_course_get_course_duration',array($filters,'batch_duration'),10,3);
    }
});

1 . No user will continue to be the member of the batch .to remove the user you have remove them manually . 2 . Im sorry I could not understand this . 3 . yes you are right .

If batch duration is set to 9999 days by default it will overwrite the course duration for which the user will be added to course if user is member of batch , there is no other implications of the batch duration actually. With the above code given to you disable this overwriting you can set the unlimited duration of the batch but it wont do anything except for display purpose .