Ok, I've finally cracked my problem with the installing of any other rom than 5.11.12 on my Mi2S. This post is for future reference. I am not sure what the problem exactly was but I got a error copying system to /system error with ALL ROMS (OK except for 5.11.12). So I guess it was a partioning thing.
To solve: install miflash on your pc. Get a fastboot rom at
http://en.miui.com/a-234.html . I used the stock chinese one. Point miflash at the ungzipped and untarred package (use 7-zip for that) flash_all.bat script. Reboot phone into fastboot with Power button and Volume down button. Flash the stock image. Boot it once. This takes forever! I am not kidding, 20 minutes at least.
Then reboot into fastboot once more. Make sure you have a custom recovery.img (TWRP or CWM) and ADB/Fastboot working. Issue the following commands:
fastboot oem unlock
fastboot flash recovery recovery.img
The recovery.img should be in the same directory as your fastboot.exe (assuming you're in Windows).
Reboot into the custom recovery, pull the battery for that and use the Power and Volume Up button to boot into the custom recovery.
Then upload a ROM of your liking (SMIUI probably) to the phone and flash that via CWM or TWRP. After that, reboot. You'll have to wait a long long time before it boots completely. This is normal. Be happy.
Took me a long time to finally arrive at this point. I hope someone will have use for this post.