UPD:
Tried to flash Chinese 5.0.2. stable ROM via MiFlash tool with flash_all.bat - SIM1 out of service but camera is OK.
Then wiped all except storage via Fastboot/TWRP and flashed EU 5.0.2. stable ROM - SIM1 out of service but camera is OK.
So the downgrade is stll impossible. Will get back to EU 5.1.1. developer ROM - I prefer working SIMs and can put up with low FPS issue.