How do they promise a 100% chargeback system?? Honestly, no one, but the CC company can promise you that at all. The only thing google can do better at the moment is pretty much on support. Google checkout has a lot less customers/transactions going on compared to paypal. Therefore, they have more time to take care of issues that arise. If they do become big like PP, their support will most likely become such as PP's or worse.

As for the gmail being tied, I believe it is so. I've used other google products eg. analytics and it also ties into your gmail.

Just make a new gmail account under your company name and you'll be fine.