Is a double barrel system less accurate long range than a single barrel setup because of the complexity that's involved in splitting the air into both the barrrels in the double barrel setup.

Actually, the overall difference in accuracy is not as bad as you might think. Generally speaking, the doubles will show a little larger shot pattern, and the pattern will be a bit more elongated in it's vertical dimension than a single shooter. About 12%. But the difference is largely related to the guns sensitivity to the consistency of the paint. Any paint Max effective range is also a very few feet shorter with the doubles.

What is more accurate overall? A single barrel Hurricane or a double barrel (over/under) Hurricane?

That's hard to say. A lot would depend on your shooting method/style. I tend to focus on shooting only one barrel and just let the other ball be a tag-a-long. Most tend to "spray" with the double and not fucus properly on their shooting.

Is it possible to toggle between single barrel firing and double barrel firing if I have the double barrel Hurricane or Typhoon?

Toggle? No but, by disconnecting the right bolt and rotating it 180 deg. you can ignore that barrel and run paint through the left barrel as a single. Velocity adjustment would have to be lowered significantly to stay legal and safe.

Can you make a 4 barrel Hurricane/Typhoon? (barrels setup in a cube formation)

I can... but I wouldn't.

Can you make a 3 barrel Hurricane/Typhoon? (barrels setup in a triangle formation)

2 is my max. That is about all the air I can get through the valve effectively.

Would the accuracy and the range suffer from a three or four barrel setup?

Absolutely.