Your not buying a product, your leasing a service, in which they control basically all the rules.. You might be able to take legal action, but the cost is going to be on you (That's the primary way they get away with so much). I would be surprised with online gaming company, they eventually find they have a flag system, for "Old Accounts, that just purchased the new expansion", simply because they think you would create a new account and purchase everything all over. As for customer service I would say good luck with that you might get a slight chance (1%) of getting someone that does not have a denial quota they might need to meet.

Getting media attention here might be your best bet though... You might have to provide more proof however that you were not cheating or your account might not have been broken in took or used by another... Most Internet/Media is skeptical in the cheating area... If you account get hacked in most of the online stuff, it's considered your compromise too (You find there is little recourse unless you find a sympathetic ear in the company that takes time to look into your case, and already had protection like 2fa turn on)...

It's sucks, but that's why I play almost all single player, and offline like gog games if possible.

You dont buy WoW, you purchase access to use and play on their servers, I'm sure somewhere in their ToS it says something along the lines of "Blizzard Entertainment, Inc. reserves the right to revoke access to WoW servers at its own digression." It sucks that this is legal, but you're paying for a service, you're not buying a product necessarily, even if it might feel like you are. Blizzard has no obligation to unban you even if you are completely innocent, and I'm not confident they would even if they had to. Corporations are soulless only seeking the most profit they can and by reducing support staff and investing in overzealous detection algorithms to keep the majority of players happy, they're accomplishing their goals.