I have a dedicated Raspberry Pi running to my stereo - one without a display. It has a HifiBerry Digi+ which is plugged into a NAD-D3020 via digital coax. I have a separate Raspberry Pi for the touch screen because it is in a different location. There may be some audio interference from the touch screen, but I am definitely not an expert there. My advice to you would be to try it and if you like it, stick with it. But you will most likely get a better audio experience out of a purpose built, dedicated music player.
One thing I would say though. At this time, I would not recommend a USB DAC on a Raspberry Pi. You will be better off with a dedicated RPi device such as a HifiBerry or an IQaudIO device. All of the Raspberry Pis have known issues with USB DACs that can cause popping and cracking in the sound output. There are some work around techniques with specific configurations, but it is still not as good as a dedicated RPi device.
As for getting things working on Diet Pi, I am not sure how much help I can be. I am very much a Linux geek and I exclusively run Arch Linux for Arm on all of my Raspberry Pis. As a Linux geek, I use the command line to adjust the display backlight. The brightness is adjusted with entries under the “/sys” pseudo file system.
Google “/sys/class/backlight/rpi_backlight/brightness” for more information
I describe how I launch Chrome/Chromium at start up in this link. But it is very specific to Arch Linux and the comment makes assumptions about Linux skill level. I provide this information as-is and cannot provide support for it.