Firstly ... yes, the debit card is a bank card that immediately debits from your bank account (as opposed to a credit card, where you pay monthly or whatever). But I don't think there's any significant difference between a debit and a credit card as far as purchasing from Amazon is concerned.
To be honest, I don't remember whether I entered the card number before or after I registered for 1-Click. I do remember that I was obliged to enter an address for 1-Click, but I think it accepted the 1-Click registration before I entered the card number (not sure about that).
You can enter any address, in any country, and you can enter as many as addresses you like, but only one of them gets assigned to 1-Click. When you try to purchase an e-book via 1-Click, at that point it checks the country and denies the purchase of the 1-Click address is non-US.
You're right that I have to purchase via the Internet (because I'm outside Whispernet range), but I disagree when you say I can avoid the 1-Click process. When buying Kindle books on the Amazon web site, 1-Click is the only option. (Is that your experience too?)
I don't think you'll have any problem with buying Kindle books when you're in the States. If you have a gift card balance, it will use that balance without asking you. As far as I can see, that works the same both on the Internet and Whispernet.
I haven't tried using my Kindle in the US, but will no doubt do so at some point. I'll report back.
One issue I've noticed is that, although I registered the Kindle via the Amazon website, the Kindle itself doesn't know it's registered to me (which is obvious really, as it has no way of communicating with the rest of the world). I'm assuming I'll be able to fix that next time I'm in the US.