I've also grown somewhat sensitive to duplication, maybe to a painful level. But, the memmove-call from the AI writeup has duplication in there:

    memmove(args->begin_argv + extend,
            args->begin_argv + consume,
            args->endp - args->begin_argv + consume);   // ← bug

If both `args->begin_argv + consume` are supposed to be the same concept and thus the same value, I'd have a variable for it by now. Some people hate it with a passion, but something like this removes the precendence thinking, prevents modification of one and not the other and makes it easier to follow, for me at least:

    retained_tail_begin = args->begin_argv + consume
    memmove(args->begin_argv + extend,
            retained_tail_begin,
            args->endp - retained_tail_begin);

Though at that point one might also encode the entire intent (as far as I understand it) in variables as well:

    space_to_replace_end = args->begin_argv + extend
    retained_tail_begin = args->begin_argv + consume
    memmove(space_to_replace_end,
            retained_tail_begin,
            args->endp - retained_tail_begin);

Sure we can golf the names somewhat, but that code has my head spin a lot less about math and precedence.