Hi Whino,
Instead of the Part Select buttons, you can also use the display buttons F - I: Press the button next to the currently selected part (e.g. the "F" button for the Right1 part) to open the Voice Selection display for that part. To go to another part, press the corresponding button (e.g. "G") once to select that part and then press the same button a second time to open the Voice Selection display.
To go to a different Voice Category, press the upper "8" (UP) button. Then you will see all Voice Category folders. (Don't forget the folders on Page 2!) Now select (open) the desired Category folder, and then select the desired Voice for the current part.
By the way, with the E.Guitar Voice Category button, there is probably an internal contact problem with the so-called "tact switch", which is located below the button on the circuit board. Please check whether the button is restricted in its mobility due to spilled liquid (e.g. beer, cola). If so, you could try cleaning the gap between button and panel with a slightly damp cloth and possibly a wooden toothpick. The contact problem may disappear (at least temporarily) if you press this button very often. If you want to have the defective button repaired (replaced), I recommend that you have this done by an authorized Yamaha Service Center. The switches under the panel buttons are only accessible if some boards are removed. You also need basic knowledge of electronics and soldering. Therefore, I do not recommend doing such a repair yourself.
Best regards,
Chris