Beyond the :: stuff, I can only think of IPv4-mapped IPv6 addresses, where you can represent a trailing 32 bits as dotted decimal (e.g. 2001:db8::192.0.2.1). And the :: stuff also exists in IPv4 in the same way, just using dots instead of colons.

ULA is equivalent to RFC1918.

LL also exists in IPv4 (PIPA), but I take your point that it's not common in most environments.

Yes, privacy addressing is different.

But, the context that you were commenting in was about the representation of addresses, not the semantics themselves ("what is a valid IPv6 string"). And there doesn't seem to be any greater complexity other than the IPv4-mapped IPv6 addresses thing. Which doesn't seem all that complex, especially if you see it as a tradeoff to escape the DNS name ambiguity of IPv4.

No, because in a context where you'd have a port number, the address is surrounded with brackets: [2001:db8::]:80