Well, I can't say much more than what I already did on the first post. I've tried a lot of methods to install the recovery, but none of them work. I even tried what Zorbeyd posted here, but it didn't work neither.
The results are always similar. Either the phone boots normally (ignoring any key combination I press), or it goes to a black screen after the Sony logo. I'm always able to go back flashing the stock rom with Flashtool, but I wanted to try KitKat, and not having a recovery prevents me from doing that...
PS: I tried method 1 on the thread Zorbeyd linked. Should I try method 3? I didn't, because I don't know how to hange those permissions on the folders. Could you give me some direction?