Ok, I'm starting to get my head around this!

I read the rules... a couple of times. I obviously have a lot to learn about the airline industry!

Just a few lingering questions and I think I will be ready to book!

1) I get the LHR transfer thing, no problem. My question is: can I change my itinerary after I have started? Assuming that I stay within the rules, can I shift around my destinations? (I didn't know if "inventory" was the same as itinerary)

2) If I am staying in the same continent, can I backtrack to previous stops? For example, if I fly into SYD, then go up to CNS, but have to go back through SYD to get out to PVG is that ok?

3) Finally, I get that I have to book all of the flights before I fly international for the first time, but after that, if I leave them "open" when do I need to book them? Can I just walk in day-of and take a flight (as long as space is available)? Who will I be booking with? Specific airlines in the airport, a central desk to call, or somewhere on the internet? For example, this EZE-SYD, how often does that run and how soon would I need to book that?

You guys have been extremely helpful in figuring out this (seemingly) unneccisarily complicated product! Thank you so much! I don't know how I would have gotten this information from the official websites...