I finally succedded reflashing the 2 IMEI codes with this tool :
http://snwritetool.com/download/sn-write-tool-v1-1728
and the usual VCOM USB drivers used for flashing ROMs
You also need the official rom to select the databases (MDDB_InfoCustomAppSrcP_MT6757P_S00_MOLY_LR11_W1630_MD_MP_V14_P14.EDB and APDB_MT6757_S01_alps-mp-n0.mp5_W17.32 files)
Of course also need the IMEI codes from the box...