Can the harmonic balance come loose?

Seems to me if the front of the crank was broken the oil seal would be leaking badly.
The fact that it doesn't make noise in park then does make noise in drive does suggest the thrust bearing. Thrust bearing wear allows forward back movement but not the up/down side to side movement. That movement requires moving pistons and everything else so it wouldn't be easy to do by hand. Just strange...
I would have to get that balancer off and know its not the problem.
Here's what you can try... Get a impact grade socket and a good long 3/4 pull bar for the balancer bolt. Wedge it in such a way as it cannot move clockwise. Bump the starter. That should break the bolt loose so you can back it out. Pray that the balancer will fall off the crank snout. If it does, you can suspect the bolt was too long and bottomed in the hole. That would allow the bore in the balancer to fail, slowly but surely.
Hope and pray for the best and be prepared for the worst is all any of us can do.