The best thing I found out of this whole guide, was your way of installing Oracle's Java 7. The commands you gave are by far the EASIEST way to install it, and I give you kudos for that. Sun's tutorial on it was way too long and hard, and caused some people to bug up their system.
Hey, I found some information that indicates some changes in Ubuntu 13.04, dealing with Java. Apparently because of legal issues, or some other thing that I wouldn't care about if it didn't affect my Minecrafting, it is now MUCH more ideal to use the "Open JDK" (the one you can find in the software marketplace) program for any of your Java endeavors on Ubuntu, including MC. I know JDK stands for Java Development Kit, and that's what it's for, but it does have a built in Runtime Environment.
I discovered this information after spending about 4 hours d*cking around, trying to get everything working on my girlfriend's computer (given, I've never used Linux, have limited command line/coding experience, and had nothing in my stomach but 20 ounces of strong instant coffee). I was using Oracle Java 7 JRE, and it worked just fine, but I was having trouble putting it onto my unity bar (even though that's still a pain in the ass with the Open JDK program), and couldn't get it set to the default running application (which is still a pain in the ass with Open JDK).
*NOTE* You can still use the regular Oracle Java 7 JRE, but apparently the Open JDK program works just as well and is easier to get up and running, and will probably progressively be the more user-friendly option as Linux pushes forward in development.
There was only one source that gave me this information, and about a million that misled me, so if I can find that particular page again, I'll come back to this thread and post a link. It might save somebody some effort down the road.