1) What is the least number of axioms needed for a practical language?
2) What is the least number of axioms needed to write an evaluator for the language in the language itself?
As I demonstrated, you need a lot of axioms to support practical programming, because practical programming involves I/O, threads, sockets, exceptions, etc.
Trying to find the smallest axioms necessary for I/O is a cool idea. But any I/O axioms will be intimately tied to the hardware, and the hardware is currently more C based than Lisp based. So the result won't be very elegant.
If you ignore practical programming and I/O, and only care about mathematical elegance, then McCarthy's original Lisp is already a quite good answer for question number 2.
Arc is quite a bit more elegant than most other programming languages, but at the end of the day it is still a practical language.
So my question to you is: what are you looking for?
I'm wondering first, exactly what axioms pg settled on. And I'm also curious as you've described it, if the hardware were built for the language and rather than the language being built for the hardware, "what is the least number of axioms need for a practical language".
I can't really answer that. Somebody would need to write an "eval" function in Arc. That would give you a pretty good starting point for figuring out how many axioms you need.