I must confess I put a lot of time in these renderings. It's a personal, non commercial project. I didn't have to comply with a deadline.
1. Yes, i tweaked the hdri, as you can see in the attached file > but only gamma.
In post production I used object masks to adjust saturation of the walls and sculptures
2. No, i didn't use a ray switch material. Corona devs and super users may now intervene...I pushed the highlight compression and contrast up to "3" or even "5".
3. Before i began to work on this project I studied a lot of images on google from the gipsoteca.
I was looking for a desaturated but still yellowish room atmosphere. I guess the floor, the hdri and the LUT may have helped to get an overall warm mood.
The most difficult part for me was to achieve the grunge on the walls, sculptures and concrete/black metal parts.
I am new to 3d rendering. Maybe i did wrong a lot in case of setting up the scene or tweaking materials. But I'm a photographer (primarily architecture), too...and this influences my cgi work a lot > camera angle, composition, colours, contrast, depth of field etc.
But at the end it lies in the eye of the beholder...design is subjective. ;o)