Follow-up/resolution to my previous post.
I was able to confirm that my three Light Gray cartridges were "faulty" by checking them in another Z6100. I installed a new Light Gray cartridge in my Z6100 and it liked this one. So that means that (before I swapped out the ink supply assembly) my printer was not only rejecting cartridges, it was corrupting good ones!
So, with the new Ink Supply Tubes/Trailing Cable assembly and a complete set of new cartridges, I manually initiated the priming process and it worked fine. We're back up and running.
If I may be allowed a quick rant though, it really burns me that I had to spend so much time and money on a problem that had nothing to do with the printer's ability to print. The problem was with the printer's cartridge sensing system which HP will tell you is there to ensure quality, but which I believe exists primarily to make it hard for owners to use non-hp inks or cartridges that have passed their "expiration" date. And it's ludicrous that I had to replace the entire Ink Supply Tubes/Trailing Cable assembly to fix this when I believe that all that was needed was the lower ISS PCA. My older Designjet CP series printers didn't have/need a "smart" cartridge system and I ran them for over ten years without a single service call. (I did my own maintenance, belt replacements, etc.) My Z6100 is less than two years old.
I'll also mention what a pain in the neck it was dealing with the HP Parts Store. First they said they had the part in stock. Then (five days later, when I was expecting the part to arrive) they informed me that it wasn't in stock. So I ordered it from another supplier. Then HP told me they DID have it. I told them to cancel the order because I had already ordered a different one, and they said I couldn't because it had already shipped (which it hadn't), so I had to cancel the second order. I did, finally, get the part from HP, but let me put in a plug for DECtrader.com. They were terrific to work with, they really knew their stuff and they know how to deal with HP! I'll go to them first next time.