I am not really sure, you’re description is confusing me a bit.
If you are getting a sub field of a group field in $_POST then you would use
$_POST['acf']['field_XXXXXX']['field_YYYYYYY']
where field_XXXXXXX is the group field and field_YYYYYYY is the sub field.
For the repeater, you need to loop over the repeater the same way you would loop over it anywhere else, or loop over the array that is returned when you use get_field().
