Here is how I did the render: There are 43 lights in this scene so I did not go and multiplied each one of them by 8. That would have been very hard to do for several lights because Blender does not allow to increase the light energy beyond a certain point. And beyond that point, I would have needed to play with the distance and with the inverse square falloff (I did not verify that but I assume you are using the inverse square attenuation on your lights except for the sun), it complicates the issue quite a bit.
Anyway, modifying the lights seemed like a too big task. So what I did, instead, is that I kept all the lights (except for the sun) as is and I added a Curve node in the composite nodes and I multiplied the whoile render by 8 with the help of that Curve node before applying the Gamma correction. This is a perfectly valid way of achieving the goal.
As for the sun light, I increased its intensity quite a bit because it was really not strong enough in the initial setup. The initial render of that scene was not only very dim but showed a sun that was totally lacking energy. So I corrected that in addition to setting the sun color to yellow.
Yup. I see that too. But I have no problems with that. I’m pretty sure that if I were to take a photograph of a real room lit only with a sun and the sky with an aperture wide enough that I could see the room illumoinated the way it is in the render, then the window sills would be saturated with light too. If there is anything that would help concerning the windows sills, though, it is that their geometry could be complexified by adding whatever it takes to hold the glass in place. But I did not worry about that. I focused on the general lighting feeling.
and in the top right corner of the last window, the light spreads to the wall, still being completely white. Do you see this too?
The light is not spreading on the wall but rather, the shadow in the top portion of the window corner is pushing the averaging of light further outside. I may have pushed the sun light a little too high though.
Besides that. there are small white dots between the ceiling and the back wall, though I’m not sure what causes them.
Those are likely antialiasing issues caused by using the RGB Curve node instead of adjusting each light individually.
BTW, I did not save the modified blend file. I will revisit it and save it this time.