If you have PiezoFlush in your LLK cartridge, then you aren't printing gloss- correct? If so, you should have PiezoFlush in your PK cartridge and ink line as well, since that channel isn't being used, to prevent clogging/drying, so it remains in good working condition if you want to use it in the future.
The ink cartridge levels will go down at different rates, depending on the images you're printing, and the amount of each shade used for printing. The LLK channel will only be used when you do cleaning cycles or print nozzle checks, and as long as you are selecting the correct "K7" curves, it won't be used when printing images thru QTR. Keep an eye on your LLK cartridge and let me know if it continues going down faster than the others. If it's not leaking or have obvious signs of a problem, then I assume it's working correctly.
I hope this helps. Please let me know if you have further questions or there;s anything else I can help you with.
Best regards and happy printing~ Dana