OK, you can wipe /data in TWRP or flash "userdata" with Flashtool.Ok, so, I've managed to re-flash the firmware, I had an error in Flashtool: "ERROR - Processing of system.sin finished with errors.", but I continued with fastboot, flashing boot and recovery (again successfully), and now my phone is in a boot loop! Yay! I've launched TWRP but I don't want to mess things up again.
I will try to explain the problem: if you have unencrypted the internal memory when following the guide and after you flashed and booted on a stock boot.img the partition is encrypted again, the process can take hours to be done (Xperia screen), the partition is not empty like on first boot "out of the box" that's why it takes a long time. If you break the process by forcing shutdown the partition will be corrupted and the phone will be stuck on init (Sony logo) because "usedata" partition is not mounted.