How can label nodes be conditionally inserted into TikZ paths?

MWE:

documentclass[border = 10pt]{standalone}

usepackage{ifthen}
usepackage{tikz}
usetikzlibrary{calc}

begin{document}
begin{tikzpicture}[every node/.style = {align = center}]
    newcounter {lfpnode}
    coordinate (LFP) at (0,0);
    newcommand {drawaline} [3] {
        ifthenelse{equal{#2}{}}
            {deftdsdabovespec{}}
            {deftdsdabovespec{node[above] unexpanded{{#2}}}}

        coordinate (LFPS) at ($(LFP) - (0,{#1})$);
        ifthenelse{equal{#3}{}} {
            deftdsdnewLFP{LFPS}
            deftdsdbelowspec{}
        }{
            edeftdsdlfpnode{LFPNarabic{lfpnode}}
            deftdsdnewLFP{0,0 |- tdsdlfpnode.south}
            deftdsdbelowspec{node[below] (tdsdlfpnode) unexpanded{{#3}}}
        }

            % this doesn't work
            deftdsddataflowspec{[blue] (0,0 |- LFPS) -> tdsdabovespec tdsdbelowspec (5,0 |- LFPS)}
            expandafterdrawtdsddataflowspec;

            % neither does this, although I didn't expect it to
            draw [blue] (0,0 |- LFPS) -> tdsdabovespec tdsdbelowspec (5,0 |- LFPS);

        coordinate (LFP) at (tdsdnewLFP);
        stepcounter{lfpnode}
    }

    newcommand {drawalineone} [1] {
        deftdsdnewLFP{LFPS}
        coordinate (LFPS) at ($(LFP) - (0,{#1})$);

            draw [blue] (0,0 |- LFPS) -> node[above] {ABOVE1-1ABOVE1-2} (5,0 |- LFPS);

        coordinate (LFP) at (tdsdnewLFP);
        stepcounter{lfpnode}
    }

    newcommand {drawalinetwo} [1] {
        coordinate (LFPS) at ($(LFP) - (0,{#1})$);
        edeftdsdlfpnode{LFPNarabic{lfpnode}}
        deftdsdnewLFP{0,0 |- tdsdlfpnode.south}

            draw [blue] (0,0 |- LFPS) -> node[above] {ABOVE2-1} node[below]
                (tdsdlfpnode) {BELOW2-1} (5,0 |- LFPS);

        coordinate (LFP) at (tdsdnewLFP);
        stepcounter{lfpnode}
    }

    newcommand {drawalinethree} [1] {
        coordinate (LFPS) at ($(LFP) - (0,{#1})$);
        edeftdsdlfpnode{LFPNarabic{lfpnode}}
        deftdsdnewLFP{0,0 |- tdsdlfpnode.south}

            draw [blue] (0,0 |- LFPS) -> node[below] (tdsdlfpnode) {BELOW3-1BELOW3-2} (5,0 |- LFPS);

        coordinate (LFP) at (tdsdnewLFP);
        stepcounter{lfpnode}
    }

    % The following causes errors aplenty:
    %    "Cannot parse this coordinate."
    %    "A node must have a (possibly empty) label text."
    %    "Giving up on this path. Did you forget a semicolon?"
    % drawaline{0}     {ABOVE1-1ABOVE1-2} {}
    % drawaline{1.3cm} {ABOVE2-1} {BELOW2-1}
    % drawaline{1cm}   {} {BELOW3-1BELOW3-2}

    % Works!
    drawalineone{0}
    drawalinetwo{1.3cm}
    drawalinethree{1cm}
end{tikzpicture}
end{document}

The general idea is simple: drawaline draws a line and accepts three arguments. If the second argument is nonempty, a label appears above the line. If the third argument is nonempty, a label appears below the line.

The vertical spacing of the lines is such that a space defined by the first argument appears between the line currently being rendered and the lowest point (including label, if present) of the previously-rendered line. This isn’t crucial for the MWE, but I’ve included it since it’s how the code is used in the original context, and it might conceivably affect which fixes are possible.

The drawalineone, drawalinetwo, and drawalinethree macros are hard-coded versions of drawaline. They function as expected. The actual drawaline macro does not. Note that neither of the approaches used in drawaline works when unexpanded{{#2}} (and likewise for #3) are replaced by simply {#2}, or by begingroup #2endgroup. In short, I don’t know how to get TikZ to recognize the content following draw as a parseable TikZ path spec.

Finally, please note that the drawaline macro must accept multi-line labels.

Any help would be appreciated.

documentclass[border=10pt]{standalone}
usepackage{tikz}
usepackage{ifthen}
newcommanddrawline[3]{
    % the position of the line is #1 below the previous line
    % it is not accounting for the (not typeset yet) above box
    path (lastline) ++(0,{-(#1)}) coordinate(thisline);
    % draw the line and mark midway
    draw (thisline) -- ++(5,0) coordinate[pos=0.5](midline);
    % if we have the box above, draw it
    ifthenelse{equal{#2}{}}{}{node[above, align=center] at (midline) {#2};}
    % if we have the box below, type it. Either case, re-set
    % the (lastline) position accordingly
    ifthenelse{equal{#3}{}}
        {coordinate(lastline) at (thisline);}
        {node[below, align=center](tmp) at (midline) {#3};
        coordinate(lastline) at (thisline|-tmp.south);}
}

begin{document}
begin{tikzpicture}[]
    coordinate (lastline) at (0,0);
    drawline{0}{Only  above}{}
    drawline{2}{Either  above}{Or  below}
    drawline{1}{}{Only  below}
end{tikzpicture}
end{document}

    newcommand {drawalinetwo} [3] {
    coordinate (LFPS) at ($(LFP) - (0,{#1})$);
    edeftdsdlfpnode{LFPNarabic{lfpnode}}
    deftdsdnewLFP{0,0 |- tdsdlfpnode.south}

        draw [blue] (0,0 |- LFPS) -- node[above] {#2} node[below]
            (tdsdlfpnode) {#3} (5,0 |- LFPS);

    coordinate (LFP) at (tdsdnewLFP);
    stepcounter{lfpnode}
}

drawalinetwo{1cm}{}{BELOW2-1}
drawalinetwo{1cm}{ABOVE2-1}{}
drawalinetwo{1cm}{ABOVE2-1}{BELOW2-1}