CS6 PMS Book Colors only as LAB

manwith2shoes wrote: ... 1. The same artwork displays differently on the same monitor depending which version of Illustrator is used, ...

This statement is not technically correct. It is not depending on which Illustrator version is used but on what Pantone library is installed. I don't have CS6 I'm still with CS5 but I have installed and use the new Pantone Plus libraries. In CS6 you can also remove the new library and install the old one and all will be the same on your computer. If you deal with others they have to do the same if you want consistency with a certain group of users.

...and the new version is vastly inferior to the old version. I don't care what monitor you're using: the area that is specified as PMS Black 6 looks like black in the CS 5.5 version and looks like a 70% gray in the CS 6 version. THAT'S NOT AN IMPROVEMENT...

That's valid only on your monitor. On mine the display of the Pantone colors is fine and PMS Black 6 looks like real black nothing like 70% gray.

... The CMYK builds from PMS colors are now different...

Again, technically this is not stated correctly although in practical terms you have a point. The two different builds have always been in Illustrator - if you choose Spot Colors... from the menu of the Swatches panel you will see the two different options CMYK and Lab. The problem is that the CMYK build and not the the Lab build was default and most people didn't bother or knew there is another option. If the Lab build was the default there would have been no problems now. With the new Libraries Pantone removed the CMYK build and keeps only the Lab build which is now default regardless what is chosen in the Spot Colors Options.

... In the past, if I sent our brand guidelines to vendors and told them our orange was PMS 173, I could be reasonable certain that their CMYK build would be consistent with other vendors. Now we'll need to explain it: "our orange is PMS 173, and our CMYK build is 0/69/100/4. Please use this CMYK build for consistency. You may notice that Illustrator will convert PMS 173 to 11.64/86.93/100/2.15, which is due to an update in the Adobe software. We prefer that you use the original CMYK build for all materials for consistency with our previous materials, and ignore what Adobe Illustrator tells you about what these values should be."...

I guess you were lucky because as I said the two different builds options were available with the previous version and any user could choose a different option. On top of that any user can double click a pantone swatch and change its build with any color model for different reasons, for example to match the color on screen with the pantone book. So, if you want to be fully certain that other users will use the same CMYK build as you,  you should have always specified these details and you don't have to add these ridiculous reasons if you want to sound professional. The simple reason is that any user can change the build as they like and this is the  reason Pantone removed the CMYK build. It was just one party's opinion about matching CMYK values that were not intended to be used the way you did.

You are a victim of an improvement that interferes with your workflow that was not based on understanding how things work. I'm not saying this because I'm not sympathetic to your situation but to explain that this is a kind of problem that Adobe will not address and the change will have to come from you.