Best Place for a Back Button on a 7" Tablet Menu

I'm currently building the interface of an Android Tablet digital restaurant menu. The user is going to be able to navigate between categories and dishes lists and navigate back as well. I'm finding very difficult to position the universal back button (which is going to appear in every screen apart from the root menu).

Here are the options that came up to my mind:

The "I" is the first that I thought about, which seems pretty obvious. But after some testing, I concluded this design does not fit very well with the tablet's shape and the way people usually hold it. To make it easy to press, I also need to make it bigger and waste a lot of room at the header...

"II" and "III" was suggested by a friend of mine, and it sounds really nice, except that it may make some people confused as it is invading the content area (I must remind that this is intended for a very wide audience, which included people with almost no knowledge of touchscreen systems)

"IV" and "V" are good, but I don't really know how bad it is that the back button is mixed with the menu buttons (like "about" and "call waiter"), maybe someone can confirm that for me.

Are there any good objective reasons to go with any of these options, or are there any other options not considered here?