I once spent a long morning discussing my faith with two elderly JW ladies, all to no avail. When they left I asked the Lord what to make of it all. He told me to read 1 Peter 2. The answer came loud and clear and I think it will help here.
1 Peter 2:4-10 (New International Version)
The Living Stone and a Chosen People
4 As you come to him, the living Stone—rejected by men but chosen by God and precious to him— 5 you also, like living stones, are being built into a spiritual house to be a holy priesthood, offering spiritual sacrifices acceptable to God through Jesus Christ. 6 For in Scripture it says:
"See, I lay a stone in Zion,
a chosen and precious cornerstone,
and the one who trusts in him
will never be put to shame." [ a ] 7 Now to you who believe, this stone is precious. But to those who do not believe,
"The stone the builders rejected
has become the capstone, [ b ] " [ c ] 8 and,
"A stone that causes men to stumble
and a rock that makes them fall." [ d ] They stumble because they disobey the message—which is also what they were destined for.
9 But you are a chosen people, a royal priesthood, a holy nation, a people belonging to God, that you may declare the praises of him who called you out of darkness into his wonderful light. 10 Once you were not a people, but now you are the people of God; once you had not received mercy, but now you have received mercy.
JWs reject Jesus, especially every "Memorial". To us who believe, Jesus is precious, but to JWs "Jehovah" is.
Jesus will be their capstone and is THE thing which causes them to stumble. "They stumble because they disobey the message" (see John 6:53-54 for example) "which is also what they were destined for".
I do not think that all JWs are predestined to stumble but at some point their eyes will open and it will be time to stop disobeying the commands of Jesus.
Blessings,
Stephen