(1) maybe, I don't fully understand what you are building/describing
(2) yes
(3) I don't know, maybe/probably, a fuse is sized to be larger (>1.25x) than the max load, and smaller than the wire used (the main purpose of a fuse is to protect the wire, it is an intentional weak point). Put another way, whatever size fuse you choose the wire must have a higher rating than it.
Only if the panels are wired in parallel, otherwise a single breaker or disconnect switch is commonly used.
Yes and yes
there are a million ways and it can be intimidating and confusing. My advice, spend a bit of time trying to understand the
concepts before focusing on the specific design. One reason there are a million ways is because a lot of things are very situational/contextual, so the design depends on the context, the use-case, and the priorities, another reason is because there are many ways to solve a given problem, and another reason is because for every valid solution (and some invalid ones) there is someone arguing its the best option, which makes it hard for a newbie like you or I.
This is why I think trying to learn the fundamentals, and the concepts is so important. If you can learn just on a basic level, the theory of how the system works, and what components do, and how energy and electricity work, it empowers you to understand a lot more about how to design whats right for you. Most youtubers and many resources focus on the how, not the what and the why, but these latter questions are very important to understanding the how.
So circling back to your questions, in light of the above focus on concepts.
A fuse is an engineered weak point, and an 'overcurrent protection device.' Its purpose is to fail safely before other parts of your system can fail unsafely.
A busbar is just a conductor (like a wire), its useful for distributing or combining. They can be a useful organizational tool in your design.