Your next advice was to flash a Libreelec image (V88II mars) to NAND but have some questions:
1) wouldn't it be wise to try to boot from sdcard first ?
2) what's my way back to restore stock android ?
3) according to "Quick installation instructions on NAND" on Armbian forum, shouldn't i choose "Burn Armbian image via steP-nand" ?
1) Indeed, it is much safer to try booting from sdcard first, but you should be able to do so just putting the LibreELEC image on and sdcard and plugging the cord; it should boot without any further procedure from sdcard. (Armbian instead requires some different setup because it uses a different bootloader arrangement)
2) If you kept the NAND data as it was, Multitool backup and restore feature will let you backup almost entirely the content and restore will bring back Android in the very same state you left it.
To add some further details, the NAND driver in 4.4 kernel will not let you backup the idbloader part of the NAND, which is the very first 0x2000 sectors, but it also does not allow to write those sectors, so tinkering with the Multitool is generally safe.
As further info, not directly related to the answer, the idbloader is very important on NAND devices because it contains code specific for the NAND vendor, thus if you lose the idbloader, you NAND data probably becomes garbage. But once you do a backup from Multitool, the Android part of the firmware is safe.
There is also a procedure that uses the AndroidTool for Windows that allows you to burn the original firmware from the image file, I don't remember quite well the procedure, but fabiobassa probably may help you there. Note that this procedure will do a factory reset, so you lose your existing data too.
3) No, because the steP-nand procedure is just for Armbian. Armbian needs to be "relocated" some sectors farther on the flash memory because of the different bootloader I talked about in answer #1. LibreELEC does not need such relocation, thus can be burned as is: just remember to say "Yes" when the Multitool will ask you to skip the idbloader (ie: the reason I explained in answer #2: NAND kernel driver can't write those sectors).