Here is a feedback of what I did:
I followed this tutorial: www . symbianize . com / showthread . php?t=736020
I used the files for the 2.3.6 version, with the 918N scatter text and 918N recovery file.I also had to play with the pilots on my computer, and finally I could flash the recovery of my phone.
After that I only had to root from the recovery with nandroid.
I played with a few root programs, and made some mistakes with Link2SD (not to sure of what went wrong), and after rebooting, the screen got black after the android logo at the beginning of the boot... and I did not have a nandroid backup (so if you have nandroid, make sure to immediately make a backup, just in case).
I tried a lot of custom roms, but they were for 918D 918M & 918N, and none was in 2.3.6. I don't know if that was a problem, but I couldn't make the phone boot this way. (I still could access to the Nandroid Recovery)
I finally found a way to repair the phone: Alcatel provides an upgrading software for the 918, after following the procedure, I got the phone with the original system restored.
I rooted again the same way and for now everything is fine.
I don't know if it is the best way to do this (I'm far from being an expert) , but it worked for me.
Maybe my experience will help someone.