Sounds like you've got it under control then. I'm not sure how much the ES will help but it won't hurt him to stay in it so if you don't mind dosing it with every water change, by all means, go for it. It may help to keep the swelling down, not sure. Poor guy, must be hard for him. I really hope it doesn't start to interfere with his eating.
Since it's a viral infection, I think it's transmitted in a different way than most waterborne diseases (like fungal, bacterial, etc). Not sure exactly how, I'll have to research that.
I agree with you! Honestly, pet store employees should have to read some fish books or something so they know what they're talking about. And I'll probably go to my grave still annoyed that no pet store advises heaters for bettas.
How hard can it be to say to a customer, "You'll need a heater and water conditioner for this fish."?