1. The movement itself if pretty nice, but I feel like I had a different idea on how the shotgun worked, It's best used as a momentum amplifier, not like a double jump. Once I figured that out it was easier to work with. One major thing about the grappling hook is that you need to be able to detatch with the spacebar, I found it a little odd to control having to click again to detatch. This would also help comboing the grapple with the shotgun for more momentum.
2. From what little of the fog I experienced in the tutorial, it seemed fine? But I didn't get past the tutorial. The fog came in while I was still trying to get over the spikes, and I got a little frustrated dying from something I couldn't even reach yet.
3. Once you figure things out its fine, but it feels a little unintuitive starting out.
I had trouble walljumping, but that just be my own computer being slow and laggy. And in the tutorial where you reach the box to move, it's not clear that you really need ot use the shotgun to give you that last little bit of height to reach the platform, as the basic jump doesn't get you there, and you apparently can't walljump off blue stone. I also recommend having a checkpoint system, as there were a bunch of times I died in the tutorial later on, and it made me start from the beginning, which was a little annoying.
4. I can't completely say, but I'm leaning towards no, as it's already kind of difficult. But maybe as the levels progress and increase in challenge.