Turns out the answer was very simple, I needed to pass the index of the qubit to probabilities_dict rather than the qubit directly.

The last 2 lines should be changed to:

# marginal probability of flag==1 must equal P[x ≥ deductible]
# index of the single‑qubit flag register inside *this* circuit
flag_index = qc.qubits.index(flag[0])

p_excess = sv.probabilities_dict(qargs=[flag_index])['1']
print(f"P(loss ≥ 1) = {p_excess:.4f}")