Hi EndyzWayz,
1. Yes you can log into both R18.com and DMM.co.jp using the same acccount.
2. R18.com accepts only Japanese Yen. The site displays other currencies, but they are simply a "reference price". You will be billed in Japanese Yen.
3. You can purchase games, doujin, monthly subscription, Picture albums etc from DMM.co.jp. However, videos and comics can only be purchased on R18.com. If you try and purchase a single video on DMM.co.jp it will redirect you to R18.com
4. For R18.com Product names will NOT be listed. Only "GT CREDIX 0570-07-3210" or "CREDIX 0570-07-3210" will be listed on the statement.
For DMM it gets a bit more complicated as it differs by service listed below:
http://www.dmm.co.jp/help/faq/credit_html/=/ch_navi=faq#Q8
5. These are all digital products. We do not ship DVD's or offer this service outside of Japan.