I bought mine last May, I believe, or around that period -- I took advantage of the sales for Mother's Day

But I got a Wi-Fi model.
Are you still in Japan and if you're not, does the 3G work? I've done a quick search and it seems that with Japanese Kindles, the 3G connection only works in Japan, as opposed to the European and North American ones. I would guess that the 3G model is specific to Japan then, probably built to work with their own network frequencies -- which means that you cannot trick amazon.de/it/com into thinking you actually bought it from them.
As to why Amazon.co.jp doesn't allow you to remove the ads, I'm afraid there's no reason -- you just can't. It says so
here at the bottom of the page, but it doesn't get translated to English when you change the website's language, so if you don't understand Japanese you have no way of knowing beforehand.
I think at this point it might be best to send it back.