Yes, you can install an older custom ROM if you unlock your bootloader
https://xiaomi.eu/community/threads/unlock_hyperos_-global-devices.71752/
After that, you should be able to flash an older official ROM if you want
@fcarrico The instructions are in the first post of the threads.
https://xiaomi.eu/community/threads/hyperos-1-0-stable-release.71170/
https://xiaomi.eu/community/threads/hyperos-2-0-stable-release.74372/
Yes, if they "open sourced" their modifications on closed source applications, they would probably have issues with Xiaomi really fast (let alone the security issues with the community publishing updates).
I am not sure what you are looking for, but you can keep track of the Xiaomi EU module releases here : https://xiaomi.eu/community/threads/xiaomieu-module-app.70890/
You say that your phone is not a Chinese version, and not a global one. Which one is it ?
Did you check which region your account is attached to on https://account.xiaomi.com/ ? Does it match the region your phone is coming from ?