 Solution

Interference CP violation concerns the relative phase between direct decay and decay after mixing. For a common CP eigenstate final state, it can occur with both $|q/p|=1$ and $|{\bar{A}}_f/A_f|=1$, so neither of the first two mechanisms is needed. In that limit $|\lambda |=1$, butproduces different interference terms for $\lambda$ and ${\lambda }^{-1}={\lambda }^{\ast }$. With $\Delta m=m_h-m_{\ell }$,The term proportional to $\mathrm{Im}\lambda \sin (\Delta mt)$ consequently reverses sign. In the CP-conserving limit, $\lambda$ is the real CP parity of the final state in a consistent phase convention. For non-CP-eigenstate final states, compare the full coefficients in CP-conjugate rates rather than imposing this simplified criterion.