Well, this is my third attempt to post in this tread in try to help you! But it is just not being published.
Yesterday I did a render for you with the necessary modifications and I think it came out the way you need!
My original message (hope it gets published now):
So, since its holiday in Brazil, I decided to give it a try.
I think Adam and Ikari are right.
Since I don’t have the add-on for the enviroment I used a regular HDR (did not tweak it or anything: you could move it around and blow out it a little bit if you don’t want people to see the exterior).
The most important part is the interior lighting, I’ve made a simple scene with two planes with soft blue emission (3 in strenght I guess) as if they were lamps. That kind of solved your problem.
Also, too much bounces (I used the regular blender setup for this) and also used the good-old clamping (5 I guess).
I took the liberty to delete all the areas lamps (I guess it was around 10).
One more thing: I simplified the nodes in the glass and also, the PVC material hadn’t any nodes, so I put a mix with diffuse and glossy for it.
Sorry about the resolution it was with 40% of the size with 750 samples. For your client I would go higher (1000 or 1250 at least, since there’s going to be a lot more objects in the scene).
Well, I hope I could be of some help also.
Good luck with the project and hope you nail it with the client!
PS.: if I may give an advice, I think in the windows panes you would have to close them a little bit (maybe moving the rotation of them) otherwise they wouldn’t block the light as they are used for.