(a) You mention that the NVidia docs push people to use libraries, etc. to really get high performance CUDA kernels, rather than writing them themselves. My argument would be that SIMD is exactly the same - they're something really that are perfect if you're writing a BLAS implementation but are too low level for most developers thinking about a problem to make use of.
(b) You show a problem where autovectorisation fails because of branching, and jump straight to intrinsics as the solution which you basically say are ugly. Looking at the intrinsic code, you're using a mask to deal with the branching. But there's a middle ground - almost always you would want to try restructuring the problem, e.g. splitting up loops and adding masks where there's conditions - i.e. lean into the SIMD paradigm. This would also be the same advice in CUDA.
(c) As you've found, GCC actually performs quite poorly for x86-64 optimisations compared to Intel. It's not always clear cut though, the Intel Compiler for e.g. sacrifices IEEE 764 float precision and go down to ~14 digits of precision in it's defaults, because it sets the flag `-fp-model=fast -fma`. This is true of both the legacy and new Intel compiler. If you switch to `-fp-model=strict` then you may find that the results are closer.
(d) AVX512 is quite hardware specific. Some processors execute these instructions much better than others. It's really a collection of extensions, and you get frequency downclocking that's better/worse on different processors as these instructions are executed.