Hi, alkimia(?)
There are many ways to create an effective illumination solution in a scene. So is very difficult to answer that question with precision, however here are some tips that you should consider regarding each of your points:
1a: As many as you need, no more no less, but don’t add lights unless you know its function. I did a tunnel before and the best way to approach something like that is solve the first to segments and then repeat the solution.
1b: spotlights work great for directional lights like the one you have here, and remember you can use more than one in a row and use them to simulate direct bounces from the floor and wall.
2:Check out the Ambient Occlussion shader before using very expensive tools like GI and FG. If you don’t have nothing animated on the scene you should check about caching the GI solution.

3: you may want to remember that more often than not a “fake” or non-naturalistic solution is more realistic and appealing to the viewer. Think in the mood of the scene and read some books on illumination for 3d graphic programs. I don’t mean this in a bad way, and I apologize in advance if I am wrong, but for the look at the images it seems that you still lack some knowledge on the theory behind lighting solution.

Hope this helps.