The initial question (how would you create a form for handling creating multiple dependent resources without using js) - is inherently about various types of state - and if/how to share them between the client and the server.

As I mentioned, I think complex business flows like these can be a great reason to go from "web page" to "web app" - precisely because it enables rich, ephemeral client state with (some) business rules close to the client.

But that doesn't mean you can't do powerful things with just html/http and some form of continuations etc on the back-end.