Wow. You've tried a lot of renders. This is a common problem so you're not alone. Let me start by explaining some of the Renditioner issues that cause it and then maybe the concept for getting a solution might be easier to grasp.
Renditioner has a built-in "tone mapper." It is a bit like an autoexposure on a digital camera. It looks for the brightest spots, like a glowing light against a wall, and then adjust the tone of all the colors, brightness, and contrast according to that bright spot. It tries to keep an area from "burning out" or just becoming a white blob. For most tastes, myself included, this adjust things too dark.
The consequences of the tone mapping are that if a render is too dark, and you choose to increase the brightness of a light to compensate, you could have the opposite effect: you can make it darker still. What happens is that the brighter light causes everything to be adjusted down in tone mapping, making the contrast between light and dark areas even more concentrated. So the dark areas become darker...
The key, therefore, to getting a more even lighting is to ensure that you don't have too many lights of extremely bright levels, and to adjust the settings so that the Natural Light slider isn't too far in one area or another. If it is only natural light and the slider is 100% to the left, then the sunlight is brighter, but the contrast is stronger. If the render is all artificial, then it depends a lot of how powerful each light is.
In some situations it is easiest to get a balanced look by making sure there are no lights that are really strong, then try one or two quick, small, preview renders (set to Lighting Preview) and see if adjusting the Natural Light slider can balance out the natural and artificial effectively. Maybe have the slider in the middle, then try with it one or two ticks to the left. You might also like trying a different lighting condition like Cloudy, but then moving the lighting slider one or two more ticks to the left.
If you can get a rather balanced light, it may still be too dark. Often the fastest way to get the light levels right are to then use a bit of quick post processing. I find that I can brighten the image in Photoshop or The GIMP quickly by using the LEVELS command (rather than brightness/contrast).