Thanks for your elaborate response and the diagram (it looks crystal clear
).
I'm afraid EDL is new territory for me, so I would need some time to figure that out.
I expected the decrypted
update.upx to contain a stock
recovery.img but that does not seem to be the case.
Anyway, thanks again for your time. Maybe I should stop spending mine into this minor firmware upgrade, because the device itself works just fine for me.