fastboot responds, except that it won't flash my recovery image
Fastboot is so unpredictable, ouch. Try the following steps.


Check if your cable is the stock one. If not, at least make sure it supports file transfer in fastboot mode.
Check that USB drivers are installed properly
Try to rename the recovery file to just "recovery.img", I know it looks stupid, but it did the trick for me.


Finally,

* the device won't finish booting (gets stuck)
* recovery mode is gone. If I try to boot into recovery, the Mi logo stays there forever

yeah, obviously it doesn't boot, you erased the recovery partition.

Also, I think for fastboot flashing isn't really needed to erase the partition you are going to flash.