Unable to install ROM with adb sideload or other methods on LG G3 d855
So I was finaly able to solve my problem :


Apparently the failure of adb sideload at exactly 47% is a known bug on TWRP, so I would assume that it's the same bug bitting me on Cyanogen recovery.
Anyway, if the recovery doesn't allow to install a ROM, there is the download mode (volume UP and plug in an USB cable).
If download mode doesn't work, try it with another USB cable (such as the one that came with the phone)
Have a Windows install available either physically or in a VM as LG couldn't bother supporting other OSes.
Follow this tutorial to reinstall the original LG ROM, but at step 7 tick the "Board DL" box as not to get stuck on "Factory Reset Status 2" such as explained in this reddit thread (doesn't seem to be a problem for everyone).
Reroot the phone and reinstall a ROM that isn't full of LG bloatware.


(Note for self : next time buy a device that hasn't been designed to work against it's owner)