Inky, I thought your reply was beautiful and I'm sure he'll recall your words one day and know that he is welcome back home when he comes to his senses.
I really don't think "warning" his future wife would really have been of benefit and it would have come off badly. She is probably a JW robot just like him and she probably would have been proud of the letter and would have taken "mild" and "industrious" to be compliments (and if someone prefers a milder personality and a hard-working person, aren't they compliments?)
Hopefully marriage goes well for them and hopefully they come to their sense. You've done all you can and now you just have to be patient and hope.
Path