Actually I think the problem comes from:

\ifdefined\Shaded\renewenvironment{Shaded}{\begin{tcolorbox}[frame hidden, sharp corners, interior hidden, borderline west={3pt}{0pt}{shadecolor}, enhanced, breakable, boxrule=0pt]}{\end{tcolorbox}}\fi

Comment that out and it works. It may have something to do with parsing of the environment options in square brackets: removing [frame hidden, sharp corners, interior hidden, borderline west={3pt}{0pt}{shadecolor}, enhanced, breakable, boxrule=0pt] also makes it work.

Interestingly, changing

\ifdefined\Shaded\renewenvironment{Shaded}{\begin{tcolorbox}[frame hidden, sharp corners, interior hidden, borderline west={3pt}{0pt}{shadecolor}, enhanced, breakable, boxrule=0pt]}{\end{tcolorbox}}\fi

to

\renewenvironment{Shaded}{\begin{tcolorbox}[frame hidden, sharp corners, interior hidden, borderline west={3pt}{0pt}{shadecolor}, enhanced, breakable, boxrule=0pt]}{\end{tcolorbox}}

also makes it work!

For completeness, the error message is:

Error at "t.tex" (line 245, column 2):
unexpected end of input
expecting \end{snugshade}

OK, the root cause, I think, is that \ifdefined is not something pandoc currently understands. Changing \ifdefined\Shaded to \iftrue fixes things.

It would be difficult to accurately implement \ifdefined, because at best pandoc would know about control sequences explicitly defined in the file it's parsing. We could limit it to those, but then we'd get some false negatives. Still, that may be better than simply failing.

TODO? Implement \ifdefined