Reflection or shared secrets is a coin toss here, I think what you have is okay. An alternative name for the package private class in java.net is SocketSecrets, just a suggestion as SocketsUtil sounds more general that it is.
I agree with Chris on throwing InternalError if any of the reflection code fails as that would indicate a bug or build issue somewhere.
jdk.net.Sockets mostly look good. I guess I would avoid _ in variable names but they are private so minor. Some of line lengths are a bit long in the method declaration. Not a big deal but they might have future side-by-side reviews harder.
Looks like "{" has ended up on the wrong line in a few places, NetworkPermission for example, a few other classes too.
ExtendedOptionsImpl.<clinit> could use a lambda I assume. ExtendedOptionsImpl.c L95 - alignment? I don't have any other comments beyond the comments on the 9 version. -Alan
