Answer:
1 - The black and white image provides contrast and our eyes are immediately drawn to it, additionally the shopping cart is close to the middle of the cartoon suggesting that it's a subject of interest to the brain and thus we question why it's on top of a hill alone, why does it have a sign reading "Safe Way" on it, and who left it there in the first place.
2 - We can tell the two figures are mountain climbers because one of the is carrying a pickaxe and the other seems to have a backpack, presumably with supplies in it.
3 - The second climber's eyes are quite wide and his hands are covering his mouth, most likely as he gasps. He is visibly in disbelief at what he is seeing, and his face is being read as such because of the psychology behind it. It's not uncommon for someone's eyes to go wide while covering their open mouth (because that's what happens when you gasp) this is a natural human reaction, so therefore it will appear in art and other mediums and we can read the moment as such.
4 - Without any context or before hand knowledge, I can only assume that either the last mountain climber left it because they tried to use it as a "safe" way down and died because of it or perhaps they didn't even die and they just never used it (because why would they), or this mysterious random cart at the peak of a mountain is just there to serve as a "safe" and "easy" back down. Most likely the character who's surprised is thinking the first option though, and I don't know about anyone else but I'd be pretty shocked to find that someone tried to ride their way down a mountain too.
5 - Simple answer to this is "There are no shortcuts, once you start something and accomplish it the only easiest way to "go back" is to go the way you came, down a path that you already know and traveled."
