BT and video recording are not that important to me (plus the tablet is for toying around) so I tried to adapt the 2.0.5.6 update instructions to 2.1.3.7 with "ssu re 2.1.3.7" and using "http://sfos.scanf.su/onda-v820w-v3/2.1.3.7" for adaptation-onda. Seems I didn't know what I was doing since "version --dup" couldn't find all the needed packages. I had nothing to lose so I even tried enabling adaptation0 & adaptation1 without luck.

Maybe an overkill, but in the end I re-ran the 2.0.5.6 update instructions and after rebooting the tablet was working fine as it was before my little experiment.