I will respectfully disagree with Stephen 
Vithanco, my background is mostly C/C++ and I still had a very similar reaction to you when learning the code base. In 2020 I deleted as much of lib/vmalloc as I could and tried to reduce other malloc duplication across the tree. Let me try to answer your questions…
Some of these have slightly different purposes. There’s the three library functions Mark noted, then the “g” prefixed wrappers that exit on failure and the “z” prefixed ones that return pre-zeroed memory. As for why the macros like N_NEW are repeated in various parts of the code base, I assume when this code was written #includes were somehow expensive and to be avoided.
My interpretation of the prefixes is “g/G” = “global”, “N_” = “new”, “z” = “zeroing”. This is terse, but pretty common in C code bases. I’ve seen the same named functions elsewhere.
Yes. They all allocate from the same heap, with the exception of lib/vmalloc (“vm” prefixed functions) which is an arena allocator. In the long term I would like to remove it completely.
Maybe. Some of these wrappers are useful. E.g. the zmalloc wrapper helps write more readable code. The “g” prefixed wrappers are also useful because many Graphviz allocations do not check the return value. None of this is really good practice, but we have to deal with the situation as it is.