MattTheMan: About the 2nd question, I increased the Final Gather rays till 2000, and nothing happened. The problem was the occlusion material I was using on the floor and walls. I increased the Samples and I obtained better results.

mcFleury: Thanks for answering! I'll use a directional light for reflections, let's see what happens.


Here is the last render: notice how the grainy texture is practically gone: