I want to keep your issues separate and I have asked you under a separate posting about your tests concerned. On the POSIX point, setting out the assembly output of test cases for Itanium, first without -pthread and then with -pthread, and showing the false sharing would be sufficient, as -pthread is supposed to switch on the POSIX memory model in gcc and it will quickly become apparent if it doesn't.
This posting is just to comment on your observations on the C11 memory model. You are quite right in saying it represents accumulated wisdom, because its starting point was the (comparatively under-specified) POSIX memory model for multi-threaded programs. However, if you read the lkml postings in questions, you will see that the kernel community do not want the generalized C11 memory model for the kernel. They want the preclusion of false sharing; they don't want full sequential consistency in all cases because of its efficiency implications.