How can I improve the text wrap in these tikz arrows?

I commented the lines of your code without deleting them and just underneath, I put the new code. If you need an explanation, just say so, I'll explain more fully.

documentclass[border=5mm]{standalone}
usepackage{tikz}

usetikzlibrary{shapes.arrows, positioning}
usetikzlibrary{decorations.text}
tikzset{->,>=stealth}

begin{document}

begin{tikzpicture}
        node (A) at (1, 0) {$A$};
        node (P) at (4, -4) {$P$};
        node (C) at (9, -2) {$C$};
        node (Z) at (9, 2) {$Z$};
        node (D) at (4, 4) {$D$};
        node (D') at (-2, 4) {$D'$};
        node (PL) at (-2, -4) {$PL$};
        node (L) at (-7, -2) {$L$};
        node (L') at (-7,2 ) {$L'$};
        draw (A) to (P);
        draw[densely dotted] (L) to node[above,rotate= 13,black]{scriptsize Some text here}(A);
        draw (P) to node[below,rotate= 21,black]{scriptsize Longer comment.}(C);
        draw (C) to node[above,rotate= -90,black]{scriptsize Short comment.}(Z);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% ---- old path ------------------------------------------------------
%        draw[bend left =15, densely dotted] (PL) to node[ above right=of D,rotate=45,black]{scriptsize A useful remark here.}(D);
% ---- new path decorated with the text «A useful remark here» --------
        draw [bend left =15, densely dotted,-latex, postaction={decorate,decoration={raise=3pt,text along path,text align={right,right indent=8mm},text={|scriptsize|A useful remark here}}}] (PL) to  (D);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
        draw (D) to (Z);
        draw (A) to (D);
            draw[densely dotted] (A) to node[above,rotate= -13,black]{scriptsize Some more text.}
(C);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% ---- old path -------------------------------------------------------
%        draw[blue,bend right =15, densely dotted] (D') to node[below right=of A,rotate=-45,black]{scriptsize More text here, too}(P);
% ---- new path decorated with the text «More text here, too» ----------
        draw [bend right =15, densely dotted,-latex, postaction={decorate,decoration={raise=3pt,text along path,text align={right,right indent=10mm},text={|scriptsize|More text here, too}}}] (D') to  (P);  
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%      
        draw (PL) to node[below,rotate=52,black]{scriptsize Text here,too.}(A);

        draw (L) to node[below,rotate= -21,black]{scriptsize Some more text.} (PL);
        draw (D') to (A);
        draw (L') to (D');
        draw (L') to node[above,rotate= 90,black]{scriptsize The last bit of text}(L);
end{tikzpicture}

end{document}