Well known to people who are knowledgable in all the intricacies of the build tools used for systems programming, which I'm pretty sure is a much smaller group than you're assuming here.

Also depends on the linker. This is the behavior of bfd/gold, usually the default linker on Linux but lld allows "back references" as it targets more platforms and many of those don't care about the ancient UNIX convention.

Yes, sort-of, though the "imperative" linker flags -( and -) may help you. if you can figure out how to quote them properly (or use --start-group / --end-group)

I did not know that but that does suck. The typical contract is that it shouldn’t matter beyond being able to parse it.

I believe it would be that way, if not for performance. The linker keeps track of unresolved symbols, and resolves those symbols while parsing subsequent object files. So if you list a dependency before it is used, that dependency won’t be linked at all, resulting in unresolved symbols — thus the need for object files to be listed in a particular order.

Edit: and the need for listing a dependency twice is to resolve mutually dependent object files. If A depends on symbols from B and B depends on symbols from A, you can link A,B,A or B,A,B (per the aforementioned reasoning).