I agree, this does not look like a colour gamut issue, although your initial description of the problem seemed to point in that direction. However, I still maintain that if you don't have any "reference" images that you can print and compare with prints from a time when the printer was working properly, you cannot necessarily rule out the possibility of a glitch somewhere else in your workflow. If you do have such images, do they seem to print well as far as you can ascertain, whether or not you have previous prints to compare with?
"If I *didn't* have a Care Pack, I could buy the part and replace it myself. I've done so before. I *PAID* for this Care Pack so I wouldn't *have* to do this. I've gotten to the point where I'll no longer recommend HP printers or HP service for anything but hobby usage, since they can't be counted on to help get a production customer back up and running". I have and use several printers, including Epson wide format, but the only HP I have is a desktop for docs, so I cannot opine on the pros or cons of the printer you refer to. However, if you are indeed capable of installing the part you have referred to, why not try that approach, regardless of the Care Pack? It seems that you are getting nowhere fast with HP service, so if you're bent on using this printer it might be worthwhile to DIY. In the end it's about being able to service your customers, is it not?
Gary