Then I'm not sure why you're asking questions about stock ROM here, without at least disclosing it.
The switch is present on our ROMs, but on stock launcher it's only visible for Japanese KDDI builds.
Works fine here. When enabled, a button shows on the left side of the screen when pressing the volume button while audio is played from a certain app or apps.
MediaFire (where you uploaded the fix) doesn't expose the date. The modem.img there might be outdated, regardless of region.
You should not have included it there at all unless an exact compatible ROM version is specified.
All requirements to see this setting are met.
You can try to log with this command:
adb logcat -s MainlineModuleControl
Do that while accessing the "Security status" screen.
Don't worry yourself with meaningless details. The way we pack the ROMs doesn't preserve modification date.
https://xiaomi.eu/community/threads/hyperos-2-0-stable-release.74372/post-744208
There are some crazy mental gymnastics in your post.
If Global ROM works for your needs, then use Global ROM.
Why would you sell the phone and watch just because something doesn't work properly with a custom ROM? Makes no sense.