I'm scratching my head at most of it and there's large portions that are incomplete or trivial. bash is ubiquitous. If you're trying to develop an entire web framework to run on a Bourne shell from busybox, dash, or FreeBSD 4, there's probably a better way to do it. Use the tool you have, not a hypothetical one compatible with 1983. YAGNI.

Here's a common pattern for appending to PATH system-wide once:

    # /etc/profile.d/foo.sh

    DIR=/opt/foo/bin

    case ":$PATH:" in
      *":$DIR:"*) ;;
      *) PATH="$PATH:$DIR"; export PATH ;;
    esac

    unset DIR

If zsh or bash is available:

    # $1 var
    # $2 prepend this string to var
    # $3 separator (: default if omitted)
    # doesn't remove or prevent duplicates
    prepend() {
      eval "export $1=$2\${$1:+${3-:}\$$1}"
    }

    # $1 var
    # $2 append this string to var
    # $3 separator (: default if omitted)
    # doesn't remove or prevent duplicates
    append() {
      eval "export $1=\${$1:+\$$1${3-:}}$2"
    }