So, have I finally managed to address what you asked?
OK, here's what I think.
One principle of Arc is to find the minimum combination of code that implements the language. So we shouldn't end up with both a 'join and a 'append; either we decide we want 'join to be able to produce a dotted list, or else we should just use the old 'join.
One way of getting succinct code is not to implement features that no one uses. So if the only reason for using 'append (or extending 'join to be able to produce a dotted list) was to pendantically pass some Common Lisp unit tests, then I be inclined to go with case (A) and just use Arc's current 'join.
However, another principle of Arc is not to forbid things to programmers unless there's no way to provide them with logical consistency. I can't think of any reason to tell you not to use
arc> `(a ,@'b)
(a . b)
if you want to... so why should I advocate for restricting splicing to lists?
It's shorter than your append2/append, at the cost of not providing an explicit error message if a non-list is passed in some argument not the last.
I just wanted it to be done in a single sexp... Unsure whether this is an Arc bug
In Arc, the entire expression is macro expanded and compiled, and then evaluated. So in
arc> (do (mac a () 33)
Error: "Function call on inappropriate object #3(tagged mac #<procedure: a>) ()"
what's happening is that the 'mac form, when evaluated, will create a new global macro 'a. However, when the 'do form is being macro expanded / compiled, the 'a macro hasn't been defined yet. So "(a)" compiles into a function call on 'a. Then, when the 'do form is evaluated, 'a is set to be a macro, and then 'a is function called. Which produces an error, since a macro object isn't callable.