A. You can turn it into a blank horizontal postcard by taking a blank white sticker and putting it on the lines.
B. Put the address in such a way that it's distinct from the content. Remember that after all the people that handle the mail are people, they can process mail that doesn't fall exactly into predefined templates, but they will not get out of their way to understand what you meant. So as long as you differentiate between the address and the message in a clear way, you will be fine.
Horizontal postcard differentiate between the address and the message with a line, and the address is usually on the same side as the stamp, so I would recommend putting the address in the blank space above the lines, and putting a big line that will separate it from the message.