"But honestly they ARE good humans, and I feel sorry that they are destroying their life and children with this ##"
The very fact that they ARE destroying their children's lives demonstrates that they are NOT "good humans." Also consider that they will let their child die before allowing a life-saving blood transfusion.
AND, if anyone leaves their religion for ANY reason, these "good humans" will act as though that person never existed, even if it's one of their own family.
"My question is, are these people so brainwashed that they are unable to think anything without the watchtower?"
Yes, they are that brainwashed. Read some books about cults -- you'll recognize the braindead dub profile immediately.
"Is it possible to plant small seeds of doubt into their mind?"
You can try, but don't hold your breath waiting for them to change.
"Or should I just say thanks but no thanks and let them figure it out themself (doubt that will ever happen)"
If you enjoy baiting dubs, keep going with the study. At the very least, the time they spend with you is time they don't spend with someone who might actually buy into their crap.