First of all, don't be discouraged that the 1912 stuff is complex. A curator at the Museum of London tells me she has a heck of a time working out how costume of that period works at all, and they're just trying to fasten and unfasten existing dresses!
Second, I absolutely agree that a costume can be wildly successful even if it is never completed. I've found that ALL of my biggest learning experiences in the bridal business were the ones that went horribly wrong and crashed and burned - the more spectacularly, the better.
I'm never completely happy with any garment/style until the second or third go.
YES. I realised some time ago that I had effectively made the same dress multiple times in the past, trying to get happy with the style, and I didn't even know I was doing it.
Fourthly, no Ultimate structure is built without building blocks. You're very much on the right track, IMHO.
Fifthly, inner structure first - yes. Like a skeleton onto which the pretty stuff is built!
This is a great post. Well done for some great insights.