= Solution
The pseudoscalar vertex replaces $-i\lambda$ by $g\gamma^5$, so
$$
S_{fi}^{(5)}=(2\pi)^4\delta^{(4)}(k-p-q)
g\bar u^s(p)\gamma^5v^{s'}(q).
$$
Using $\gamma^5\not q\gamma^5=-\not q$ with the conjugation sign gives
$$
\mathcal P_5=\frac{g^2}{2}M^2.
$$
The interactions are experimentally distinguishable through the decay rate and its threshold behavior: scalar decay is proportional to $(1-4m^2/M^2)^{3/2}$, whereas pseudoscalar decay is proportional to $(1-4m^2/M^2)^{1/2}$. Spin and parity correlations provide further discrimination.
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