I also topfill. Part of my process is to keep the vent and outlet hole sealed during refilling. I refill until the clear plastic tab at the top of the ink reservoir side touches the ink. I install the silicone plug on the reservoir...THEN...
...I remove the outlet seal and squeeze the...
Yours truly removed the outlet clip of a new OEM cart before removing the serpentine vent cover (yeah, it's possible, just not easy)...AT 7400' ALTITUDE! The cart must have been filled near sea level. It equalized pressure....OUT THE OUTLET (what a mess)!
I can agree that Canon print heads have a design lifetime, probably based (loosely) on nozzle firing count, else they won't make it through the warranty period. I doubt that there's an actual "nozzle firing" register in the firmware that, when reached, initiates some sort of disabling function.
Canon pint heads "die" for several reasons.
The most common (IMO) is the extreme current duty imposed on the microelectronics and microconductors in the print head nozzle "die" (no pun intended). To increase the life of a thermal (eg, Canon) print head:
PRINT SLOWER! (use "night" or "quite"...
The only sure way I know of to get single carts to print single swaths is through a nozzle check. "Inputting" RGB color levels has to pass through the driver, which then can mix inks in a proprietary way.