I was able to figure out how to disable the diamond button action in the dual screen unit. It comes with compromise though. You lose access to Radio app, Audio app, AV IN app and Video app. All these apps are useless anyway, except Radio if you use it. I'm still looking at potential workarounds for that, but for now, my workaround is radio streaming app like iHeart radio. I rather have the diamond button not changing source in the unit, hence I'm OK with this for now.
Instructions:
1. Select settings and select Car Infotainment.
2. Scroll down and select About Device
3. Select the Gear icon on the top right near first line that starts with HMI
4. Input code 8888 and hit OK
5. Click on Menu Icon
6. Click on Radio, and then click again to remove it. Note that Radio will delete from the block and appear on the left column. You can later revert this action (if you want to go back to normal) if you click on the block you want, and then select Radio.
7. Repeat same process with AV-IN, Audio and Video
8. Click Save on top right corner
9. System will Reboot
Once the system reboots it will disable the diamond button function in the dual screen unit when using CarPlay or Android Auto. Now you can use the diamond button to control functions in the car (like activating Sport or Sport+) without worrying about changing source in the unit.
Note that if you are not using CarPlay or Android Auto, using the diamond button will activate the Bluetooth Screen. This is because we can't deactivate that application, otherwise we will not be able to connect devices
When CarPlay or Android Auto is active, the Bluetooth app transfer all services to these apps hence the Bluetooth app can't be opened.
Hope this helps some of you. For now, I have not found a way to assign media functions to the diamond button, hence, we can't use it to skip track yet. However I'm still looking for a solution on this. But, deactivating the diamond button (even with these compromises) is a way forward.
I used macrodroid to try to start Radio Activity, which is inside the com.ts.MainUI package and it has the following package activity name: com.ts.main.radio.RadioActivity. This opens the radio but it can't be operated. It is just stuck with no information on that screen, which I'm sure is due to the fact that the app is deactivated.
More to come on this, but good progress in the past few days...
Let me know if you have any questions!