On the demo, the non-linked menu items are like placeholders where I simply need a title to be a blank link that activates the sub menus. But this and how the menu functions should not be affected. What is odd here is that it works on your site except when on your tablet in landscape orientation (Portrait it goes to the mobile menu).
The submenu that goes to your guestbook should work…I watched your video again and in the first part, when you first touch the guestbook link on the submenu, the browser is showing that it’s spinning (it’s waiting to load the page) as I can see the address bar has the spinning timer. Perhaps you have to wait a bit longer for it to load?
If this doesn’t work by waiting longer for the page to load, I’m not sure what I can do because there are so many different types of mobile devices, I cannot develop and physically test themes in every single one. Here is one example of how some mobile devices, including the OS can affect a function in a website. My Celestial theme menu (which is based on the same menu as the default WordPress theme Twenty Thirteen), was not functioning in the iPhone 5. I discovered that it was actually the ios7 (the new version) was causing issues and made the submenu links white. I rebuilt the menu, and then it was fine. I was lucky enough though that I have an iphone 5 so I was able to test it in a true iphone environment.
Let’s try a test…I just now made a page and used your guest book form code to make it so it’s as close to it as possible, and added it as a submenu on my Contact link on the live demo site. Then try it out on your tablet and see if it works for you.