A potential solution to posting a commutative diagram is to include it as an image. This can allow you have diagonal arrows. :) Here's my workflow for creating commutative diagrams in LaTeX using TikZ. A minimal working LaTeX document looks like this:
C \arrow[d,hookrightarrow] \arrow[dr,hookrightarrow,bend left=50,dashed] & &
Y \arrow[r,hookrightarrow] \arrow[d,twoheadrightarrow]
& X \arrow[d,twoheadrightarrow,dashed]
\arrow[dr,twoheadrightarrow,bend left=50] &
B \arrow[r,hookrightarrow,dashed] & Y' \arrow[r,twoheadrightarrow,dashed] & A
Then you can compile this document however you do, zoom into your commutative diagram, take a screenshot, and crop the image.
Or you can avoid taking a screenshot and cropping if you have the other appropriate tools. On my Linux OS I can do this at the command line. I've used this technique on my website, where it's better to have the diagrams be PNG images with transparent backgrounds. For the PDF of the diagram named
pdftoppm -png -r 600 main.pdf > white.png
convert white.png -fuzz 10% -transparent white transparent.png
600 corresponds to the resolution of the image
See also these questions on other Stack Exchange sites.
\longrightarrowlooks a bit better, but I haven't tried). $\endgroup$