Ah ! That's why I didn't see anything then ! I thought it was done for the moment, and only the shorting would rescue it.
Turns out that it began to automatically boot in Maskrom mode. However, I thought it was broken because operations like upgrade_tool RFI or upgrade_tool WL didn't work anymore.
In fact, in maskrom mode, WL only works after a successful DB operation. You really have to do operations in order, else you get "operation fail!" messages again and again.
IF the machine boots with the 2207:320a id AND Nothing appears on the serial console
THEN you can restore it by cloning the miqi-prebuilt git repository and using the flash.sh script (or do what it does, if you're not using machine capable of running a shell script...)
However note that any WL operation WILL fail before a successful DB operation.
Anyway, thanks everyone for these tips. I was able to restore a U-boot and recreate a partition table on the eMMC. I'll see what to do from there.
@benn If you can add these informations to the FAQ, and create a thread named "Help ! I think my MiQi is dead !?", copy the FAQ in it and pin it, this would help people tremendously. Nothing is more stressful than searching a solution to such problems randomly, without knowing if you'll find useful informations.
@peba Thanks for the tip ! I think this should also be added to the FAQ !