The fact that the errors are all over the place in regards to which printer/computer/OS is having the problem or whether you map with by hand or via GPO makes me think that this isn't a Group Policy problem. I've been Google'ing around and have probably found the same stuff you've found. I found a KB article that might be worth a try:
"Windows cannot connect to printer" error message when you try to create a Point and Print connection to a remote printer from a Windows 7 or Windows Server 2008 R2-based client computer.
I am getting the specified module not found error when Group Policy preferences tries to install the printer. I can manually install the drivers without issue now that I have the Point and Print GP setting actually applying on the client. There are also 64 TCP/IP printers specified in the GPO where I was seeing the specified driver cannot be found message. Our print server is located at our CoLo facility and I am considering setting up a server here at HQ to test this GPO and see if the issue is the speed with which the print drivers are installing.
What kind of printers are these? I saw you mentioned Canon earlier. Is that the only brand you're using? [Sorry for asking the obvious question that I'm sure you've already tried, but...] Are you using the latest copy of the driver for both the x64 and x86 driver on the print server? If the vendor offers both PS and PCL, have you tried using the opposite of what you are using? Have you tried going back a version on the driver?
I have printers from Xerox, Sharp, HP, Canon, Konica, and Toshiba. I am using PCL drivers mostly and have not tried switching the drivers from PCL to PS (this is not even an option with some printers). I am using the latest drivers from the websites for the respective manufacturers.
At this point, if you've tried all of that, I think you're at the point of calling vendors. Since you can generate the error(s) by manually mapping the printer, you should be able to get someone at the printer manufacturer to help you out. You might also want to consider giving Microsoft a call.