If you want to get packets fast into userspace you can now apparently also AF_XDP on Linux which is also effectively a kernel bypass. The driver will put packets into a ringbuffer before they hit the remaining kernel, and userspace can fetch them effectively there. I haven’t benchmarked for the same application, but it promises to give similar levels of performance.

I think the other questions are more ones of library design and less about the packet API one uses. On top of all kinds of APIs one could build singlethreaded state machines which deque packets and process them. Or other architectures if required.