@Mario Thank you for the detailed reply. I am familiar with HPLip as we use it on our other Linux-based computers in-house. My organization is testing Endless for use in our non-profit computer award programs, so I am throwing as much stuff as I can at it to see what works and what doesn’t, including every printer that I can find. We have had good luck with HPLip and Epson, but poor results with Gutenprint. Thanks for the links - I’ll search there first from now on before trying to install a new printer.
As to my original problem, I don’t understand what you mean by removing the printers “manually” as root. I’ve tried removing the printers via CUPS (in the web browser) but that hasn’t worked either. Can you please give me a quick tutorial?