I was there to see La Trophee, don't remember fog-- just a gray day-- but record breaking storms were going on elsewhere in France. There are views everywhere. There are lots of sunny days, even in winter. Enjoy.
Edit: I've gone through there dozens of times because it's the road between my SIL's house further down the mountain and the autoroute entrance. It's sunny most of the time, even in winter--which is why so many painters go there for the light.
As for finding the beginning of La Grande Corniche--it's Nice. I made a wrong turn getting onto the Moyenne Corniche one day just as I was leaving Nice and ended up on La Grande Corniche. The road drops down to around the altitude of the Moyenne on the outskirts of Nice. I have no idea how I got there--just a wrong turn. I didn't see the views because I was driving and we wanted to get back to the house, but my husband enjoyed it. The family usually takes the Moyenne Corniche or the autoroute for going back and forth, so I'm rarely on La Grande Corniche itself.