I'm OK with all of it - I get it and see completely why she would be attracted to this guy. What I'm not OK with is that she won't acknowledge it to me. It means I can't trust her and will never see the real her. That's the problem for me.
If you want to explore a tangential thread - go find under General Relationship thread my thread I did on "Anyone here Poly?" It's NOT about swinging.
It makes me think of it because well, you obviously have the fundamentals/foundations of your relationship down okay - solid finances, lots of sex. And while it's very easy for all of us to become distracted by the death of your child, well, it's NOT ABOUT YOUR CHILD. It's about you, as a couple. Not everything CAN nor SHOULD be about your loss/him/her.
You both haven't come to terms with the fact that on some level you are polyamorous - capable of loving, or at least having a crush on someone else. This is by YOUR own admission as well.
So. . .well. . .I am totally NOT suggesting you live some kind of "weird hippy lifestyle" but understanding how you are wired with YOUR crush and understanding how your wife is wired with hers, which you seem to get, and somehow figuring out TOGETHER how you want to act on it, or not act on it (usually not seems to be the way) would be therapeutic. She needs to come to terms with it and is probably having some amount of self-loathing in that she is wired this way.
This is assuming that everything as grand as you posted, and I have no reason to doubt you.
The enemy here appears to be shame.