I finally found some time today to play with this file. My purpose was to find a good compromise between quality settings and render time. My render time was 2 hours and 15 minutes and I think the final result is not too bad. At least, there's no grain and there's no strange shades of green, which were your main complaints.

I removed all your trees because I had problems with two or three of them. I just used the standard palm tree and coconut tree. They don't look very nice but for my purpose, above, it was ok.

I changed several things in your atmospheric settings. The most important was that I replaced GI by AO. I found no significant difference in the tests I made, for this particular scene. Another very important change was that I removed softness from the sun because from the tests I made, for this particular scene the softness didn't have any significant effects and was slowing down everything

tremendously. I will make another post with a screenshot of the atmospheric light settings.
Quality boost of sky fog and haze: you had 12, i used zero.

I did not use a second sun (or directional light, as Vue calls it). Neither that nor sky dome lighting gain. I found that the simple haze was sufficient to light the shadows if the sun shadow density was set at 90%

You used the "closeup ground pack" for the base texture of the mountains. That's a huge texture file (1500x1000), suitable for close-ups, not for distant terrains (specially if they're invisible, like in this case). It unnecessarily slows down everything. I replaced it by a procedural, fast grass simulation. It's invisible anyway.

Notes on render settings:

You had optimize volumetrics light on in the render settings. That's not a good idea because "optimize" means less quality. Anyway, in this particular scene this didnt matter because you had no volumetric lights.

You had advanced effects quality at 100%. That's way too much specially because there's no significant advanced effects here. You only need that in very special situations. To put it at 100% in this scene is just a waste of render time. I changed it to 46%.

You had texture antialiasing. There's no need for it in this scene, the aliasing doesn't come from the textures here, obviously.

Your render settings were unbalanced. You had excessive subrays but an insufficient value in the quality slider.
You had min subrays per pixel at 81. That's excessive, IMO. I set it at 8.
You had max subrays per pixel at 156. That's also excessive. I set it at 20.
On the other hand, AA quality was too low, which explains a lot of the grain. You had it at 85%, i put it at 95%.