= Solution
With B nearest the observer, <light> from the reflecting target near the observer first crosses Q and then P on its way to the mirror. <Unpolarized light> remains unpolarized through Q, then becomes x linear after P. Mirror <reflection> preserves that linear orientation. On return it passes the same P without further ideal rejection and then Q, becoming circular toward the observer. Thus \b[the reflected target remains visible].
Algebraically, with input and output at the observer side,
$$
J_{\rm return}=QPM P Q=-QPQ,\qquad
\operatorname{tr}(J_{\rm return}C_{\rm in}J_{\rm return}^\dagger)=\frac{I_0}{2}.
$$
The target here is used only as a source/reflector of ordinary <unpolarized light>; no physiological model is involved.
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