Is shooting a rifle "careful coordination of several limbs" and therefore uses the average of the two limbs? I know very little about guns so maybe RL knowledge of someone else could help here....

Yes, I believe that falls into the group of tests where it would be averaged.

You could shoot a rifle one handed, but you'd be taking penalties to do so... even the shortest carbines are really built to be handled with two hands.

Of course, if the hand doing the shooting vastly outperforms the other one, it might end up with more dice despite a penalty than it would if you averaged your two arms out anyways...

Are pistols assumed to be fired with one hand? If so, that would mean only the attribute of the shooting arm would be considered, right?

Yes. Which is one of the reasons I ruled around things like Agility boosts for cyberarms

Based on the other thread about cyberlimbs, I thought people were trying to make them more useful.