Do not flash TWRP to boot partition, it goes to recovery ("fastboot flash recovery twrp.img").
Second command for boot TWRP image is "fastboot boot twrp.img".
http://www.miui.com/thread-16250712-1-1.html
Here is China developer roms. You can flash latest developer rom on top of stable with MIUI system updater app, it makes factory reset.
Might be best to unbind Mi Account before upgrading stable to developer so you don't need to unlock it at start...
You can check "\META-INF\com\google\android\updater-script" script in rom zip package.
package_extract_file("firmware-update/NON-HLOS.bin", "/dev/block/bootdevice/by-name/modem");
If this line is included, it updates modem firmware.
If you had Mi Account signed in before you flashed new rom/reseted it, you have to unlock it with same Mi Account. It's newish Android safety measure, same as in Google phones with Google Account.
Yes, Qualcomm version is mido/HMNOTE4X. Wipe cache/dalvik/data and install .eu rom from TWRP.
I believe you can nowadays install Xiaomis official .zip version roms with TWRP same way as above.
https://www.theandroidsoul.com/edit-build-prop-without-root-using-adb-in-twrp-recovery/
TWRP has root rights, you can rw mount /system and modify files with ADB.