Have you ever replaced the printhead on your Pro-100? If it has been used that much, I would think a new printhead is in order. I believe his website is (or was) inkjetprinthead.com. Super nice guy and very informative about printheads. If that head is still available, it will be much less expensive than a new printer.
Also, please don't dump the printer. There are parts (like the solid steel shaft that guides the printhead across the paper) that could be reused by people that are trying to keep this really great printer going!

I am curious as to what "scheme" most photographers use for backup. I have seen incremental or differential plans and daily and weekly updates. What works best for you?

The problem sounds more computer hardware related than the program. I would check drivers, hard drives, USB connections, etc. I would also look at your installed RAM and how much video ram is on your video card. Also, there are Photoshop CPU requirements with new, specific instruction sets to make PS run correctly.
My point is, it may not be Photoshop actually causing the problem.

There have been some good tips about printing your 9x19. The only thing I would add is that if you want the full 19 inch width, you will need to print, "Borderless" which is an option in the canon drive for the PRO-100.

Just keep in mind that the image will overprint the paper a bit to achieve the edge to edge print. It is possible to adjust the size of the overpint a bit in the Canon driver but I can't remember where to access that particular function.

Definitely figure out some ventilation. All the chemicals, especially fixer and stop bath, put off strong odors. I built a darkroom that I designed and I had a LARGE and deep sink that made washing easier. It was a great place for developer, wash and fixer trays up to 16x20 inches. A good source of water and 2 sinks, one large and a "standard" kitchen sink worked great.
I also had plenty of good power with multiple outlets. I used a window AC unit and had a vent fan that was specifically built for darkrooms that I could use if the odors got too concentrated.
Good luck with your project and have fun!

Ah, the muse.

I have thought about this issue many times in the last few years. I started with film, chemistry, darkroom and have progressed to digital over the years. I have been all over the place in my thinking about your question.

However, when I really dig in, all forms of photography are interpretive and manipulative. Ansel Adams had a notion of previewing an image before the shutter was clicked. He called it previsualization. It was a process of interpretation to achieve a specific goal in the final image. I think that's what we all do anyway. Whether it's before we shoot, after we shoot or while we are shooting we are manipulating electrons and protons and emotions and assumptions to interpret a scene that none of us really see in the same way anyway.

Our vision is constructed of variables that are at least, somewhat unique to each and every one of us. A photograph is NOT the actual scene. It's an interpretation of what we wanted to, or thought we saw. I'm not sure how we would agree on exactly what any of us see.

I know there are "purists" and they have their discussion points and emotions about film and they are valid and embraceable abstractions about photography. I just don't think they are any more valid than the arguments FOR abstraction, be it film or digital.

I better send this off before I change my mind, again!

Sounds like you might be printing a CMYK file. A CMYK file can produce some weird results if the printer is expecting sRGB or Adobe RGB, etc.

I have a Canon PRO-100 and it is a great printer. There is a "Maintenance" tab in the printer utility that allows you to maintain the print head. As to your "flat" color problem, it sounds like your image file is set up for CMYK printing. The PRO-100 produces the best results when the original file is R-G-B. If you use PhotoShop or a similar editing program, you can check the details of the image you are printing and change the attributes. In PhotoShop, the Mode attribute (CMYK, RGB, Grayscale, etc.) is available in the "Image" tab. Hope some of this helps.