How can I create a varied (or at least tasty) menu for a 12 day hike whilst using as few ingredients as possible?
We take home-dehydrated cooked meat (beef, chicken, ground beef) along with spices, tomato leather to make tomato sauce, pasta, rice, dehyrated potato-and-sauce (eg scalloped potatoes) from the grocery store, home-dehydrated vegtables, and dried fruit. From that you can make stew, pasta-with-meat-sauce, curry-on-rice, and so on. You might also be interested in making your own english muffins since bread products squish and don't keep well: http://www.gregcons.com/canoe/muffins.htm. 

Backpacking is tougher than canoeing but as long as you can be sure of plenty of water, you should be fine with dried products. We usually bring a little frozen meat for the first night, and frozen bacon for the first morning, well wrapped in newspaper, but those might be too heavy when hiking.