Well, leaving aside Darwin (who I think was kind of a jerk) and the science of geology, it strikes me that Genesis uses the word “day” as a unit of time, but that it’s hard to see how it can be a “day” in the strict sense if the sun and the moon were not yet created on the first day, since they supply us with the measures of time.
Similarly, the Bible says that the sun rises and sets and goes around the earth, but that is a way of speaking that we still use today, even though most of us believe that the earth goes around the sun. But to all appearances, the sun rises, travels across the sky, and sets, and there’s no reason not to speak of it that way.
Or, as Terry Pratchett might say, “It’s turtles all the way down.”
It seems to me that the underlined is a key point: to argue that 'day' must mean strictly 24-hours is to ignore Joshua's Long Day which also was a 'day', and like you point out clearly associates the sun's apparent movement with the time… but when there's no sun, how do you define it? (Obviously asserting 24-hours is, again, relying on the sun.)