Aftermarket cartridges can not only leak, they can also ruin the print head by ink starvation. I have experienced a similar leak to yours and at the same time having had ink starvation from other cartridges of the same brand.
I recommend to buy some of these windowed Canon OEM cartridges that have been converted to use with your printer by having had the chip replaced. The cartridges are cleaned and ready for use. They have a low profile silicone plug for refilling. This plug might need to be replaced by a zero clearance plug, as some canon printers have very little clearance.
See this post for more info.
Both the cartridges from Octoinkjet UK and from Precisioncolors are Canon OEM and can be used, as the windowed cartridges will need the chips from your cartridges. I would recommend the
ready to use cartridges from Precisioncolors, no chip change is needed and shipping and customs might be less expensive from Canada than from UK. Also remember to buy a resetter. Remember to specify the proper type of resetter. The proper resetter is for PGI-225/CLI-226 cartridges (North America region).
Both Octoinkjet UK and Precisioncolors sell the same brand of refill ink, Image Specialists, which is considered very good by many forum members.
The service error 6502 indicates an ink sensor error. An ink sensor is positioned under the print head carriage and shines an infrared light beam towards the prisms of the cartridges when the print head carriage passes over it. A prism that is covered with ink from ink still being present in the reservoir is not reflective. A prism that is no longer covered in ink reflects the infrared light and this is detected and signals that the cartridge is getting low on ink. There is another ink sensor that checks the state of the LEDs on the cartridges at start up. There is also info about the amount of ink stored in the chip. If the sensor is disabled by being drowned in ink or it detects a cartridge that is nearly empty although there should be sufficient ink in the reservoir, an error 6502 is given.
I haven't answered the questions about service mode operations yet, I will look for some good links and post that later. The procedure in the thread by Nifty that you linked to is correct, but it doesn't apply to your printer. It applies to much older printers.