It's why I prefer to use her superior physical abilities and aerial control than simply using eyesight. Other than Sanji, who has that level of control in the air in the Straw Hat crew? She would be able to get to the highest point of the ship (with the option to go even higher) by the time Usopp even attempts to climb up the main crow's nest. Having a designated pair of eyes to keep track of the whole area is insanely useful as that allows other members to focus more on their main duties. Both in casual sailing and in intense, all-hands-on-deck scenarios.
The way I see this scene with Usopp spotting the guards is that it is a showcase for his sniping abilities rather than an opportunity for a lookout moment.
Anyone could've seen these loud drunks in the open as Straw Hats and allies slowly move toward the entrance of Onigashima. On the second page, someone even says that they saw people as well. But the only member that could take out those guards quietly at that distance was Usopp. Other members would either be too slow or too loud. That is what I see as the point of this scene.
I'm not sure about this point either way as we occasionally haven't seen other roles represented on other crews either, especially since The Straw Hat crew is unique as they have such a small crew with individuals with designated roles. I'm actually pretty sure we've only seen Blackbeard's crew introduce members with designated roles, though I could be wrong.
You do bring up a good point but I think I can explain why this happened.
Here is the sequence from 899.
I see it as a perfect execution of strategy from a Yonko fleet.
I think it's worth noting just how quickly the pincer move started. They go from having Germa at their backs with nothing in front of them to being blocked by a fleet with a giant genie right on top of them. The only mistake Carrot made here is that she didn't catch the ships in front a little earlier as she was focused on Luffy and Sanji alongside the other members. But the second those ships made it around the island it was too late anyway.
The BMI pirates whipped around the island where no one could see, and as everyone on the ship was caught by surprise by the sudden fleet in front Daifuku took the initiative and attacked while their backs were turned, all in an instant. Anyone in the crow's nest would've been taken by surprise by this and not exclusively Carrot. It's also worth noting that CoO specialist Sanji noticed neither the fleet nor the genie as well.
There are plenty of good reasons to be confident that Carrot won't end up joining alongside Yamato, but I would say that a lack of role on the Sunny is not one of them.