Generally, if you draw land that is touching at the "points" it treats it as one continent.
Also, despite what the books say, there are no wonders that affect only a continent. The ones that say this in the book actually apply to your entire civ.
As to the question of max trade from a single city....
I couldn't help myself wondering about the max science from a single city, so I rigged up a map with 2 cities with all rivers, and 4 gold specials. Again, I emphasise, this terrain will not occur in a natural, system generated map.
I played it as basic OCC, for quite a while. Once my own land was fully improved, I went over and improved the AI city as well. I basically stopped researching when I hit computers to avoid getting flight. Eventually the AI built Superhighways and a sewer but every time they were going to grow, they built more engineers. Eventually they started growing again. Through "non-cheat-mode" play I was able to get the AI up to size 18 before they discovered flight cancelling my colossus. (I did do a few restarts along the way, but it was otherwise cheat free) This gave me 3 +85 trade routes, for 417 total trade arrows and 14 einstens. This gave a total of 3672 beakers.
Finally, I put it into cheat mode to max out the trade of the AI city by adding citizens until it worked all the squares, this bumped the trade routes up to +87 for 423 arrows and 3720 beakers.
(Sorry about the image quality. I think once the arrows reached all the way through the food box, it resized the main city screen image to be smaller within the window so the arrows could extend off the edge, hence all the extra black space.)
I'm pretty sure this is the max science you could possibly get from a single city.