hm, maybe, but i’m not sure i agree. the “well” makes sense in the context of the panel as “well then, let’s go!” and i read “it fell” as the little cat saying “i fell off!”
I don’t think you are missing anything. I see it kind of like you do, like - “By arrival, the capacity…”(and see what happens by viewing the comic).
Trying to understand each part by dissecting is good for learning, but I also find sometimes you understand better when you don’t think too hard either.
You know 着く頃には means around arriving and that the に marks that time and は marks that it is the topic. And then you can deduce that 定員 is potentially a subject or object. At that, I feel like that is enough.
I think the title is the joke- I’m pretty sure the idea is that the title is an open ended sentence, “The capacity around the time we arrive…”
In the comic the cats are falling off. I think the title is making you infer what it’s trying to say. “Our capacity by the time we arrive will be half!” Or “less” or something.
For some reason I’m reading the 「着く頃には」as describing the capacity as a preposition. Capacity is the subject, the time they arrive is the topic (は being a topic marker, not a subject marker). Maybe I’m reading it wrong, but that’s the idea I get from the title =P