I had 3 different 1084S's as well as a 4th 1084S-P or whatever the "2nd generation" ones were, and all of them adapted to PAL mode by changing the refresh rate and I never had to adjust anything on the monitor to see the full screen.
This was with NTSC A500/A1200's, so maybe in the case of a PAL machine this may not be the case, but I don't see what the difference would be, the video signal remains the same AFAIK.
No, there is no difference when using the RGB connection. I had a PAL 1084S and an NTSC 1080, and they don't sync automatically to a different screenmode but you have to adjust them manually... not a big deal really...
The only thing is, as you are probably aware, Commodore sold lots of different models, that is to say, models that were significantly different hardware-wise and even had different connectors on the back under the same model number, which means that you can never know which behaviour to expect from one until you try it... Even two 1084s which look perfectly identical from the outside might have different specs.