Hi [USER=132217]@JeffK[/USER]
I don't know of a situation where PS will change from grayscale to RGB without giving you a heads up. That would be worth getting to root cause.
I do know that if you run something through ACR on the front end, once can change what is sent on to PS that can be different than what came in to ACR.
I usually don't go into the details with the OP unless and issue crops up. I am already way to prone in provided details and not keeping it at a high enough level when a short and simple answer would be best. I agree with simplifying or in quoting Einstein: “Everything should be made as simple as possible, but no simpler.” I have a long way to go in achieving ability.
Your are right, most don't pay attention to the wide gamut monitor potential issues until they show up. So explaining how to deal with such issues will last beyond my lifetime I am sure 
As far as dither and changing of color spaces I will give an example below all of which is in 8 bit mode.
I created a perfect gradient from top to bottom in green (1 color bit change for each movement to the next pixel row) going from black to full white and it starts in Color Space ProPhoto RGB
The next box to the right is the same gradient amplifed by a curves adjustment Layer so the gradient steps can be visualized. Also, I have included the Green histogram which shows that every level of green is the same.
The next box to the right is what you get when converting to sRGB with Black Point Compensation yet no dither. You can visually see the banding and note that in the histogram there are some gaps where a certain gray level is not represented and others where it is over represented. This is the hallmark look for banding.
The next box to the right is the same except that dithering was included to the transformation. It is my understanding that dither is noise added yet the noise is limited to about 1 bit difference from its neighbors. You can see the random noise added in the image yet perceptually the banding appears diminished. You can also see in the histogram that the dither helped fill in the empty slots that existing without the dither. It turns out that the human eye/brain combination is very good about seeing edges and transitions so adding a small amount of noise can make the image to break up the banding look betters (banding looks worse visually than a small amount of noise).
So there is no right answer about dither. PS leaves it on by default as it is a lot simpler to have it and not explain why artists are getting banding and what steps to avoid the banding.
Hope you find the info helpful
John Wheeler
[ATTACH=full]120206[/ATTACH]
