I consider family obligations very serious. In this case, however, your parents are using you and your duty to family. They are using you and avoiding asking for help from the one in their cult because of his cult activity. That is actual JW training to "use the world." Total BS.
I would tell them simply that you need to know if they will change their relationship with you depending on what happens with you and the JW's- inactivity, disfellowshipping, whatever. If they say they may, will, might.... then I would tell them that my love is unconditional and that only after exhaustively asking the JW for help and coming to realize that their deatbeat JW son won't help them, to call you again and see if their answers have changed. Otherwise, you will check in on them and send them a card every now and again and let them know how you are doing and you wish them well.
But that's me.