For many years I believed what your wife believes so I certainly can relate.
If Jesus sacrifice covers a JW's imperfect faith, why wouldn't it cover others?
If Grandma believes in her heart she is living a Christian life, loves God and neighbor, but is Lutheran, what makes the fruit of her faith not good enough?
Grandma believes Jesus is God, so her faith is wrong .... well, is it wrong to call Jesus God? It was propesied as being a title for him (Mighty God), John 1:1 says he is God or (a god), Thomas exclaimed "my Lord My God" and Jesus didn't tell him it was wrong ... so perhaps the word trinity isn't in the Bible, perhaps the definition of trinity isn't the perfect explanation for God, but the Bible clearly uses the word God in reference to Jesus, who is closer to the truth? Would a JW feel comfortable calling Jesus God as a title?