The design of robust runaway electron avoidance and mitigation techniques for reactor-scale tokamaks must rely on experimentally validated models. A set of experiments particularly well-suited for model validation was presented in Ref. [^1], which varied the magnetic field strength and amount of injected argon in JET-ILW plasmas in order to determine the ``runaway electron existence domain'', within which substantial runaway electron currents were observed. In this work we use DREAM [^2] to revisit the results of Ref. [^1]. We find that the runaway existence domain is approximately recovered when plasma current and argon amount are varied, indicating that one of the crucial processes contributing to the runaway generation is avalanche multiplication. We also extend the modelling previously done in Ref. [^3], by treating the massive gas injection via a combined edge particle source and radial transport model. While the model shows some dependence on the duration of the gas injection and speed of the transport processes, the variations are small and do not greatly affect the overall conclusions about the runaway electron existence domain. [^1]: C. Reux *et al*, *Runaway electron beam generation and mitigation during disruptions at JET-ILW*, [Nucl. Fusion **55** 093013 (2015)](https://doi.org/10.1088/0029-5515/55/9/093013). [^2]: M. Hoppe *et al*, *DREAM: A fluid-kinetic framework for tokamak disruption runaway electron simulations*, [Comp. Phys. Commun. **268** 108098 (2021)](https://doi.org/10.1016/j.cpc.2021.108098). [^3]: C. Gustavsson, *Sensitivities of the runaway current in JET disruptions to massive gas injection and initial plasma current*, [MSc thesis](http://hdl.handle.net/20.500.12380/308020), Chalmers University of Technology, Gothenburg, Sweden (2024).