When the tecnhician came to replace the defective starter ink tank, I indicated him that also non-starter ink tanks could have the same problem, while Canon warranty does not cover consumables. He said that, in such case, I should have asked for a replacement directly to the dealer. My reply was that I really can not imagine any dealer that, here in Italy, would accept it. I am still of this opinion.
As for the possibility of disabling ink level detection, I discovered (accidentally) that it is reported in the documentation. However it is not clear wether and how this can damage the printer (with air bubbles in the pipes, maybe). The relative path is ../iPF5000/eng/usersguide/html/tp000129.html
Finally... today, for the first time, I have finished a starter non-defective ink tank, which is GY; by agitating it I can tell that it is almost totally empty. In total, I have printed on 16.9m^2 of paper (65% semiglossy, 35% matte, mostly color).