I've problem with battery drain and also without WiFi, I've battery drain also if I put my MI3W in flight mode! Is not properly like your problem but maybe there's something wrong in the software... Also for me sometimes is needed press 2-3 times the toggle for activate the WiFi radio and also for me this problem is present from 4 different ROM versions. Now I'm on xiaomi.eu_multi_MI3WMI4W_V7.5.2.0.MXDCNDE_v7-6.0 but I've this big battery drain since I merged on MM (I've done it without make fresh install, just the classic wipes).
However I've read in an another forum that someone has solved the battery drain problem installing the official China Dev ROM via Fastboot (I don't remember if via Mi PC Suite or manually via MiFlash, which version and if he has erased the data and the storage or not) then made the classic procedure (TWRP via adb then flash xiaomi.eu ROM on internal memory). I haven't time for doing this (I've about 250 App and in this moment I haven't a lot of time for backup and restore all) but maybe this can help you, if you wanna try!
Someone has reported that, with this procedure, he has solved also the problem of flashing time via TWRP (if you flash the ROM maybe you have to wait a unusual time, about 15-20 minutes like a lot of people!)
keep us updated!