Why are the coordinates of nodes different from normal coordinates and how can we add ticks to an arbitrary vertical line in the coordinate system?

I need to plot the following figure in LaTeX.

as suggested I tried some stuff myself and so far I have reached the following:

documentclass[border=5mm]{standalone}
usepackage{tikz}
usetikzlibrary{plotmarks}
usepackage{pgfplots}
usetikzlibrary{arrows.meta,automata,topaths}
begin{document}
begin{tikzpicture}
begin{axis}[
  axis lines=middle,
  axis line style={-Stealth,very thick},
  xmin=0,xmax=5.5,ymin=-5.5,ymax=6.5,
  xtick distance=1,
  xticklabels={,,},
  ytick distance=1,
  yticklabels={,,},
  xlabel=$theta$,
  ylabel=Net Utility,
  grid=major,
  grid style={thin,densely dotted,black!20}]
addplot [domain=0:5,samples=2] {x*2/3-0.5} node[right]{$N (Incumbent)$}; %These are not shown completely
addplot [domain=0:5,samples=2] {x*2-4.5} node[right]{$P (new entrant)$}; %These are not shown completely
addplot +[-Stealth,very thick,mark=none] coordinates {(5, -5.4) (5, 6.4)}; %Why is it brown colored? and how can I put the ticks in this one too?
addplot +[mark=none] coordinates {(3, 0) (3, 4.4)};
addplot +[mark=none] coordinates {(4.2, 0) (4.2, 4.4)}; %Why is this blue colored?
draw (80,465)node[]{$frac{p_{N}}{Q_{N}}$}; %What do these mean? (80,465)
draw (240,465)node[]{$frac{p_{P}}{Q_{P}}$};
draw (340,465)node[]{$frac{p_{P}-p_{N}}{Q_{P}-Q_{N}}$};
addplot +[Stealth-Stealth,very thick,mark=none] coordinates {(3, 4.2) (4.2, 4.2)};
draw (365,1055)node[]{$frac{1}{p_{P}-p_{N}}$};
addplot[red,mark=*] coordinates {(0.75,0)};
addplot[red,mark=*] coordinates {(2.25,0)};
addplot[red,mark=*] coordinates {(3,0)};
addplot[red,mark=*] coordinates {(3,1.5)};
addplot[red,mark=*] coordinates {(4.2,0)};
end{axis}

end{tikzpicture}


end{document}

Which gives me this:

But there are some problems with my attempt (I have also commented the problems I can’t understand in front of the corresponding line in the above code):

1. I cannot understand the node coordinates I have used to show the labels inside the plot (for example draw (80,465)node[]{$frac{p_{N}}{Q_{N}}$};) and how they turn out to put the labels. I mean it has done the thing that I wanted and it is OK. I just don’t understand it. I would expect to use the coordinates (0.75,-1) (the place where $frac{p_{N}}{Q_{N}}$ is right now) but it doesn’t work and puts the label in somewhere that makes no sense
2. (I realized this one, just add black inside the brackets and it worked)Some line segments have been colored blue or brown automatically. I like the red ones makes the plot more elegant but do not know how to change the blue and brown one back to black color.
3. (I figured this one too. Had to change the xmax to a bigger number.) The labels in line 20 and 21 are trimmed and not shown completely.
4. How can I also put ticks on the right hand side vertical line as same as the left vertical line?

documentclass[tikz, border=5mm]{standalone}
%usepackage{pgfplots}
usetikzlibrary{arrows.meta,
                calc,
                intersections
                }
%pgfplotsset{compat=1.17}

begin{document}
    begin{tikzpicture}[y=5mm,
         > = Straight Barb,
arr/.style = {-Stealth, thick},
dot/.style = {circle, fill, minimum size=3pt,
              inner sep=0pt, outer sep=0pt},
every label/.style =  {label distance=2pt, fill=white, 
              inner sep=1pt, font=footnotesize}
                        ]
%   axis
draw[arr, name path=A]  
            (-0.2,0) -- (6,0) node[below  left] {$theta$};
draw[arr]  (0,-5.0) -- (0,6) node[below right] {Net utility};
draw[arr]  (5,-5.0) -- (5,6);
draw[densely dotted, gray]
            (0,-5) grid[xstep=1, ystep=0.8] (6,6);
coordinate (O) at (0,0);
%
draw[name path=B] 
            (0,-0.5) -- (5,2*5/3-0.5) node[right]{$N$ (Incumbent)};
draw[name path=C]
            (0,-4.5) -- (5,2*5-4.5)   node[right]{$P$ (new entrant)};
            
% intersections
path [name intersections={of=A and B, by=ab}]
    node[dot, label=below:$frac{p_Nvphantom{-}}{Q_N}$] at (ab) {};
path [name intersections={of=A and C, by=ac}]
    node[dot, label=below:$frac{p_Pvphantom{-}}{Q_P}$] at (ac) {};
draw [name intersections={of=B and C, by=bc}]
    (bc |- O) node[dot, label={[xshift=0.7em]below:$frac{p_P-p_N}{Q_P - Q_N}$}] {};
%
coordinate (d) at ($(bc) + (0,4)$);
coordinate (e) at ($(d)  + (1.5,0)$);
draw[densely dashed]
    (bc |- O) -- (d) 
    (e) -- (e |- O) node [dot] {};
draw[<->]  
    ($(d) + (0,-0.5)$)  
        --  node[label={[yshift=-1ex]$frac{1}{p_P-p_N}$}] {} 
    ($(e) + (0,-0.5)$);
             
    end{tikzpicture}
end{document}