1. The grapple has a good amount of force, but I actually think a way to make it fun is to have even MORE force, and make the volcano wider so you have more room to zoom around. That way it's a little more chaotic, but in a good way because after playing for a while I easily learned the mechanics and it felt pretty easy to me. I wanna feel like spiderman zooming around really fast if that makes sense. Really fine tune the physics to make it smooth.
2. I remember your group wanting to have a candy theme, so I think something like candy cane ledges, that break apart after you grab onto it would be cool. Or, maybe a gummy surface that's sticky and makes it harder to jump or reach the next grapple point. Again, I would either widen the volcano or scale down the player so you have more room to add stuff and make it flow nicely.
3. A few times I died immediately because as soon as you start the lava is right under you, and it was a little annoying starting out when you are trying to learn the controls. But, after that I think the lava is waaay too slow, since you can jump ahead of it pretty quickly. So, I think there should be a short wait at the beginning before the lava starts rising, but after that it should be fast in my opinion.