Most vets aren't actually trained to deal with "exotic" pets, and for some reason this includes birds. Any good avian trained vet will recommend quarantine, no matter what. It's a small step that can save your other birds, especially if you have an entire flock.
For now, I would remove Spot and put him in his own area of the house for at least 30 days, and has a clean bill of health.
The drops that you are using are a good preventative measure against mites, but if Spock indeed does have an infestation, he will likely need more than just one treatment. (Also, the vets at this store should have seen this!) Please take him to an avian vet for a better diagnosis and treatment plan if needed.
You'll also need to clean and sterilize BOTH bird cages, top to bottom, including toys, perches, food and water dishes, everything. Mites can burrow into wooden perches, hide in tiny cracks, hang out in chew toys. Do this when you give the birds the drops, that way you'll kill off any potential pests.