We report numerical simulations of positronium experiments designed to measure the
n
=
2
fine-structure intervals. The simulations include all possible interference effects between all 20 states in the
n
=
1
and laser-excited
n
=
2
manifolds as well as representations of the electric and magnetic fields present in the waveguides used in the experiments. We find that rf wave reflection from the vacuum chamber walls is a possible explanation of previously observed line-shape distortions and shifts. We also characterized several systematic effects, including those arising from quantum interference, that are likely to be significant for future measurements
