  Accurate modelling is necessary to support precision experiments
investigating strong-field QED phenomena. This modelling is particularly
challenging in the transition between the perturbative and nonperturbative
regimes, where the normalized laser amplitude $a_0$ is comparable to unity and
wavelength-scale interference is significant. Here we describe how to simulate
nonlinear Compton scattering, Breit-Wheeler pair creation, and trident pair
creation in this regime, using the Monte Carlo particle-tracking code
Ptarmigan. This code simulates collisions between high-intensity lasers and
beams of electrons or $\gamma$ rays, primarily in the framework of the locally
monochromatic approximation (LMA). We benchmark our simulation results against
full QED calculations for pulsed plane waves and show that they are accurate at
the level of a few per cent, across the full range of particle energies and
laser intensities. This work extends our previous results to linearly polarized
lasers and arbitrarily polarized $\gamma$ rays.
