1. Not always. Also, depending on the road there might be a physical road, and a seperate road reserve. They don't always correlate. Although they're meant too...
2. If they are on a current (registered) plan, they will be a public road
3. Correct. There may be access restrictions by State or Local Government action.
4. It will be noted on the Land Title if it's a state action. NSW government (in their wisdom) sold off their title register to be administered by private firms 'more efficiently' so I'm not sure the current process (i'm QLD based.) Local Governments should have it in a Public Gazettle document, but sometimes it is just signs or deterrent fencing.
5. It could be held just in the Local Council registry. It could be a statement burried in an official reply letter to a specific individual (with a copy stored somewhere.) You could find it with a Right to information request.
Example: Duck Creek Road in Lamington QLD. Though that's only for Vehicles just as an example, you can walk it. For years there was little/no information other than some vague replies by Scenic Rim Council on Facebook posts asking if it was open yet.
Also this "Not a road but also a road" that I'm currently reinstating/defining at the moment.
https://i.imgur.com/AAE6Q7B.png The road exists, it was created and dedicated as new road. It's legally a gazetted road. Just it's exact location (size, area, position) was never officially defined by a Surveyor. It's a Sealed Bitumen road, that gets 100's of cars over it each day, but doesn't yet exist where it exists... legally speaking.
Survey plans are held, in QLD, by DNRME (soon to be renamed to DNR) and you can access them at your local Titles office. QLD is pretty good with open data though. So I'd start on QLD Globe with the road parcels layer, or download the entire Cadastre;
https://www.data.qld.gov.au/dataset/cad ... and-series