Switching between external audio card and monitors

[PGJ]

Hey guys,

after installing Retro Pi 4.0 RC2 and updating all core packages (especially the mupen64 emulator) everything runs smooth on my Raspberry Pi. Nevertheless a lot of fine tuning by editing config files was necessary to set everything right. My Pi will be used as a stationary retro console but I would also like to use it on the go. Thus I purchased a 5" 800x480 Waveshare LCD (including a case), a powerbank, an original PS3 bluetooth controller, an external USB sound card (since the audio jack on the Pi is awful) and I built myself a little LEGO housing for my Pi. All components work together well and the system can be used on the go. Nevertheless switching from the 5" screen and USB audio to my TV (including audio via HDMI) is quite frustrating since it does not work properly.

Issues:
1. The 5" display does not have an integrated scaler. The resolution detected by the Pi is incorrect (right side of the screen is not used). I can correct this by setting the resolution manually in the /boot/config.txt (https://learn.adafruit.com/adafruit-5-800x480-tft-hdmi-monitor-touchscreen-backpack/raspberry-pi-config). As a consequence the Pi does not adapt to the resolution of my TV (Full HD) automatically. Is there a possibility to let the Pi detect the correct resolution at boot up?
I already tried this tutorial (http://blogs.wcode.org/2013/09/howto-boot-your-raspberry-pi-into-a-fullscreen-browser-kiosk/; step 2) but it does not fully cover the 5" screen. Identifiying the 5" display via EDID is not possible (https://www.raspberrypi.org/documentation/configuration/config-txt.md), so individual settings for it cannot be configured.

2. I was able to activate the external sound card using this method: http://www.sudomod.com/forum/viewtopic.php?t=144. The only downside is that if I unplug the soundcard the audio will not be transmitted via HDMI nor audio jack (even after reboot and using the Retro Pi menu to switch the outputs). Furthermore as long as the USB soundcard is disconnected errors show up while (re)booting or shutting down the system. Can the Pi somehow detect if an external soundcard is connected (--> uses it) or not (--> uses HDMI or audio jack)? Or would it be possible to integrate another menu option in the audio selection menu in Retro Pi?

3. The Pi 3 supports Bluetooth and I successfully connected my PS3 controller to it. I also wanted to connect my Bose SoundLink Mini II via Bluetooth but the Pi is stuck at "Searching for devices..." if I use the Bluetooth menu in Retro Pi. Any advice to solve this?

Thank you guys so much for all your help and this awesome Retro Pi project!
Bye