I haven't dealt with visitation as my child was older when I divorced.
However, I completely agree you need a meaner lawyer, preferably one with experience with alcoholism or at the very least child abuse.
The situation you describe is quite sickening really. An 8 year old locked up in a basement with an active alcoholic?
Now is the time to step in absolutely. This is dysfunctional at best.
The Grandparent's sound like enablers of the worst kind, willing to use your child to "save" their own grown up Son.
If nothing else comes from this, I should think you should at least be able to get an order for Soberlink to be used before and during ALL visitation. You now have proof that he is currently drinking. I would ask your new, mean, lawyer to attempt to get the medical records.
I would also absolutely have it put in the order that there be zero overnight visitation and if that isn't possible, that she needs her own sleeping quarters, upstairs.
Good luck to you, please keep us updated.