As far as being better, they all usually find something the other does not (not always the case but it does happen) if there are any instability issues. Kind of like anti-virus and anti-malware applications.
OCCT = LinX = IBT. They all are the exact same. Usually, you want to run LinX for at least 25 runs or 30 minutes at the highest possible memory usage.
Memtest86+ checks for RAM stability.
P95 is not necessary after you do LinX, but you can always use it for a long term stress test (5-6 hours or more, i.e. overnight). Do small FFTs for maybe an hour, then large FFTs for overnight purposes.
Not necessarily, Linpack isn't very stressful on the memory subsystem. I've also passed some 50 runs of IBT only to have smallfft show errors with my old wolfdale and maybe yorkfield (I'm not 100% sure on that one).
So, no Linpack is not the end all be all. It is a good test but not the only test that you should use to determine stability.