With regard to (2), to destructively append the list '(1 2 3) to xs, you can:
(zap join xs '(1 2 3))
"zap join" is several characters longer than ++, but zap has general utility.
I use the string function to concatenate strings. It seems to work identically to +, as long as the first argument is a string. I do give heterogeneous arguments to string usually, and I like seeing that they will clearly be coerced into a string.
I have a couple of ideas.
1. It would be little problem for me if Arc provided + as is and provided a "plus" function that worked only on numbers and allowed me to simply go (= + plus) and proceed normally. Unfortunately, that would break all the functions in arc.arc and so forth that use + to concatenate lists (which includes, among other things, the setforms function, which is used in expand=). It would be really nice if one could "freeze" the variable references in those functions, so that changing the global value of + wouldn't change what "+" in the bodies of those functions referred to.
2. If you use concatenation so much, perhaps we could allocate an ssyntax character for concatenation. & is currently used for andf (experiment with ssexpand to see). We could boot out that usage, or perhaps have "&&" become andf. Good/bad idea? (Precedent: my memory tells me the TI-89 uses & for string concatenation.)
Regarding your second suggestion, we could also use . instead of &, as that's what Perl and PHP do - feels a little more natural to me. But . might cause me a little mental friction in differentiating between the different uses of . in (. "a" "b") and (do (= a '(1 2)) a.0).
To be honest, I'm still not crazy about the idea simply because I don't need the speed boost and + doesn't seem to cause me to use extra mental cycles when reading my code. I'd be open to it though if the community really wanted it that way.
We could vote and also ask PG what he thinks and then make a decision.